[Question]
  [
The other day we were writing sentences with my daughter with a fridge magnet letter. While we were able to make some(`I love cat`), we didn't have enough letters to make the others (`I love you too`) due to an insufficient amount of letters `o` (4)
I then found out that while one set included 3 `e` letters it had only 2 `o` letters. Probably inspired by <http://en.wikipedia.org/wiki/Letter_frequency> this would still not reflect the actual situation "on the fridge".
## Problem
Given the text file where each line contains a "sample sentence" one would want to write on the fridge, propose an alphabet set with minimum amount of letters but still sufficient to write each sentence individually.
Note: ignore cases, all magnet letters are capitals anyway.
## Input
The file contain newline separated sentences:
```
hello
i love cat
i love dog
i love mommy
mommy loves daddy
```
## Output
Provide back sorted list of letters, where each letter appears only as many times to be sufficient to write any sentence:
```
acdddeghillmmmoostvyy
```
(thanks, isaacg!)
## Winner
Shortest implementation (code)
### UPDATED: Testing
I have created an extra test and tried with various answers here:
<https://gist.github.com/romaninsh/11159751>
  
[Answer]
# GolfScript, 28 / 34 chars
```
n/:a{|}*{a{.[2$]--}%*$-1=}%$
```
The 28-character program above assumes that all the input letters are in the same case. If this is not necessarily so, we can force them into upper case by prepending `{95&}%` to the code, for a total of 34 chars:
```
{95&}%n/:a{|}*{a{.[2$]--}%*$-1=}%$
```
Notes:
* For correct operation, the input must include at least one newline. This will be true for normal text files with newlines at the end of each line, but might not be true if the input consists of just one line with no trailing newline. This could be fixed at the cost of two extra chars, by prepending `n+` to the code.
* The uppercasing used in the 34-character version is really crude — it maps lowercase ASCII letters to their uppercase equivalents (and spaces to `NUL`s), but makes a complete mess of numbers and most punctuation. I'm assuming that the input will not include any such characters.
* The 28-character version treats all input characters (except newlines and `NUL`s) equally. In particular, if the input contains any spaces, some will also appear in the output; conveniently, they will sort before any other printable ASCII characters. The 34-character version, however, does ignore spaces (because it turns out I can do that without it costing me any extra chars).
Explanation:
* The optional `{95&}%` prefix uppercases the input by zeroing out the sixth bit of the ASCII code of each input byte (`95 = 64 + 31 = 10111112`). This maps lowercase ASCII letters to uppercase, spaces to null bytes, and leaves newlines unchanged.
* `n/` splits the input at newlines, and `:a` assigns the resulting array into the variable `a`. Then `{|}*` computes the set union of the strings in the array, which (assuming that the array has at least two elements) yields a string containing all the unique (non-newline) characters in the input.
* The following `{ }%` loop then iterates over each of these unique characters. Inside the loop body, the inner loop `a{.[2$]--}%` iterates over the strings in the array `a`, removing from each string all characters not equal to the one the outer loop is iterating over.
The inner loop leaves the ASCII code of the current character on the stack, below the filtered array. We make use of this by repeating the filtered array as many times as indicated by the ASCII code (`*`) before sorting it (`$`) and taking the last element (`-1=`). In effect, this yields the longest string in the filtered array (as they all consist of repeats of the same character, lexicographic sorting just sorts them by length), *except* if the character has ASCII code zero, in which case it yields nothing.
* Finally, the `$` at the end just sorts the output alphabetically.
[Answer]
# J - 37 char
Reads from stdin, outputs to console.
```
dlb#&a.>./+/"2=/&a.tolower;._2[1!:1]3
```
`1!:1]3` is the call to stdin. `tolower;._2` performs double duty by splitting up the lines and making them lowercase simultaneously. Then we count how many times a character occurs in each row with `+/"2=/&a.`, and take the pointwise maximum over all lines with `>./`.
Finally, we pull that many of each character out of the alphabet with `#&a.`. This includes spaces—all found at the front due to their low ASCII value—so we just delete leading blanks with `dlb`.
[Answer]
# JavaScript (ECMAScript 6) - ~~148~~ ~~139~~ 135 Characters
**Version 2:**
Updated to use array comprehension:
```
[a[i][0]for(i in a=[].concat(...s.split('\n').map(x=>x.split(/ */).sort().map((x,i,a)=>x+(a[i-1]==x?++j:j=0)))).sort())if(a[i-1]<a[i])]
```
**Version 1:**
```
[].concat(...s.split('\n').map(x=>x.split(/ */).sort().map((x,i,a)=>x+(a[i-1]==x?++j:j=0)))).sort().filter((x,i,a)=>a[i-1]!=x).map(x=>x[0])
```
Assumes that:
* The input string is in the variable `s`;
* We can ignore the case of the input (as specified by the question - i.e. it is all in either upper or lower case);
* The output is an array of characters (which is about as close as JavaScript can get to the OP's requirement of a list of characters); and
* The output is to be displayed on the console.
With comments:
```
var l = s.split('\n')             // split the input up into sentences
         .map(x=>x.split(/ */)   // split each sentence up into letters ignoring any
                                  // whitespace
                  .sort()         // sort the letters in each sentence alphabetically
                  .map((x,i,a)=>x+(a[i-1]==x?++j:j=0)))
                                  // append the frequency of previously occurring identical
                                  // letters in the same sentence to each letter.
                                  // I.e. "HELLO WORLD" =>
                                  // ["D0","E0","H0","L0","L1","L2","O0","O1","R0","W0"]
[].concat(...l)                   // Flatten the array of arrays of letters+frequencies
                                  // into a single array.
  .sort()                         // Sort all the letters and appended frequencies
                                  // alphabetically.
  .filter((x,i,a)=>a[i-1]!=x)     // Remove duplicates and return the sorted
  .map(x=>x[0])                   // Get the first letter of each entry (removing the
                                  // frequencies) and return the array.
```
If you want to:
* Return it as a string then add `.join('')` on the end;
* Take input from a user then replace the `s` variable with `prompt()`; or
* Write it as a function `f` then add `f=s=>` to the beginning.
**Running:**
```
s="HELLO\nI LOVE CAT\nI LOVE DOG\nI LOVE MOMMY\nMOMMY LOVE DADDY";
[].concat(...s.split('\n').map(x=>x.split(/ */).sort().map((x,i,a)=>x+(a[i-1]==x?++j:j=0)))).sort().filter((x,i,a)=>a[i-1]!=x).map(x=>x[0])
```
**Gives the output:**
```
["A","C","D","D","D","E","G","H","I","L","L","M","M","M","O","O","T","V","Y","Y"]
```
[Answer]
## Perl - 46 bytes
```
#!perl -p
$s=~s/$_//ifor/./g;$s.=uc}for(sort$s=~/\w/g){
```
Counting the shebang as 1. This is a loose translation of the Ruby solution below.
---
## Ruby 1.8 - 72 bytes
```
s='';s+=$_.upcase.scan(/./){s.sub!$&,''}while gets;$><<s.scan(/\w/).sort
```
Input is taken from `stdin`.
Sample usage:
```
$ more in.dat
Hello
I love cat
I love dog
I love mommy
Mommy loves daddy
$ ruby fridge-letters.rb < in.dat
ACDDDEGHILLMMMOOSTVYY
```
[Answer]
# Python - ~~206~~ ~~204~~ ~~199~~ ~~177~~ ~~145~~ ~~129~~ ~~117~~ ~~94~~ 88 chars
```
print(''.join(c*max(l.lower().count(c)for l in open(f))for c in map(chr,range(97,123))))
```
I wasn't sure how I was supposed to obtain the file name, so at the moment the code assumes that it is contained in a variable named `f`. Please let me know if I need to change that.
[Answer]
## Ruby 1.9+, 51 (or 58 or 60)
```
a=*$<
?a.upto(?z){|c|$><<c*a.map{|l|l.count c}.max}
```
Assumes everything's in lowercase. Case insensitivity costs 7 characters via `.upcase`, while case insensitivity *and lowercase output* costs 9 characters via `.downcase`.
[Answer]
# R (156, incl. file read)
With *table* I construct the letter frequency table for each sentence. Then I end up with taking for each letter the maximum value.
```
a=c();for(w in tolower(read.csv(fn,h=F)$V1))a=c(a,table(strsplit(w,"")[[1]]));a=tapply(seq(a),names(a),function(i)max(a[i]))[-1];cat(rep(names(a),a),sep="")
```
Ungolfed:
```
a=c()
words = read.csv(fn,h=F)$V1
for(w in tolower(words))
  a=c(a, table(strsplit(w, "")[[1]]))
a = tapply(seq(a), names(a), function(i) max(a[i]))[-1] ## The -1 excludes the space count.
cat(rep(names(a), a), sep="")
```
Solution:
```
acdddeghillmmmoooooostuvyy
```
[Answer]
# Haskell, ~~109~~ 108
```
import Data.List
import Data.Char
main=interact$sort.filter(/=' ').foldl1(\x y->x++(y\\x)).lines.map toLower
```
The program reads from stdin and writes to sdtout.
It is quite straightforward: it breaks the string into a list of lines, and rebuilds it by iterating on the list and adding the new letters contained in each line.
[Answer]
# Perl 6: ~~56~~ *53* characters; ~~58~~ *55* bytes
```
say |sort
([∪] lines.map:{bag comb /\S/,.lc}).pick(*)
```
For each line, this combs through it for the non-space characters of the lower-cased string (`comb /\S/,.lc`), and makes a `Bag`, or a collection of each character and how many times it occurs. `[∪]` takes the union of the `Bag`s over all the lines, which gets the max number of times the character occurred. `.pick(*)` is hack-y here, but it's the shortest way to get all the characters from the `Bag` replicated by the number of times it occurred.
EDIT: To see if it would be shorter, I tried translating [histocrat's Ruby answer](https://codegolf.stackexchange.com/a/25665/15012). It is 63 characters, but I still very much like the approach:
```
$!=lines».lc;->$c{print $c x max $!.map:{+m:g/$c/}} for"a".."z"
```
[Answer]
# Haskell, 183 162 159
Assuming the file is in `file.txt`!
```
import Data.Char
import Data.List
main=readFile"file.txt">>=putStr.concat.tail.map(tail.maximum).transpose.map(group.sort.(++' ':['a'..'z'])).lines.map toLower
```
If file.txt contains, for example
```
abcde
abcdef
aaf
```
The script will output
```
aabcdef
```
Basically I'm appending the whole alphabet to each line, so that when grouping and sorting, I'm sure I'll end up with a list that contains 27 elements. Next, I transpose the "frequency table", so that each row in this array consists of the frequencies of a single letter in each line, e.g. `["a","","aaa","aa","aaaa"]`. I then choose the maximum of each array (which works just like I want because of how the `Ord`-instance of Strings work), and drop the letter that I appended at the start, get rid of the spaces, and output the result.
[Answer]
**C, 99 chars**
```
t[256];main(c){for(--*t;++t[1+tolower(getchar())];);for(c=97;c<123;c++)while(t[c]--)putchar(c-1);}
```
It crashes if less than one newline is provided. I think it could be fixed quite easily. 
[Answer]
## kdb (q/k): 59 characters:
```
d:.Q.a! 26#0
.z.pi:{d|:.Q.a##:'=_y}.z.exit:{-1@,/.:[d]#'!:d}
```
* generate pre-sorted seed dictionary from alphabet .Q.a
* process each line of input, convert to lowercase, group into dictionary, count each element, take alphabetic characters from result (I.e. prune spaces, newlines, etc at this stage) and use max-assign to global d to keep a running total.
* define exit handler, which gets passed in to .z.pi to save a delimiter but otherwise unused there. Take from each key-value to generate list of characters, flatten and finally print to stdout.
-1 adds a newline, using 1 would save a character but does not generate the output specified. Wish I could get rid of the .z.pi / .z.exit boilerplate, which would remove 14 characters.
Edit: avoid use of inter/asc by using seed dictionary.
[Answer]
## Perl, 46
```
for$:(a..z){$a[ord$:]|=$:x s/$://gi}}{print@a
```
Here's another Perl solution, reads from STDIN, requires `-n` switch (+1 to count), ties with primo's score but runs without complaints :-). It exploits the fact that bitwise `or`'s result has longer string argument's length. 
[Answer]
I'm adding my own solution:
## Bash - 72
Assumes that input is in file "i"
```
for x in {A..Z};do echo -n `cat i|sed "s/[^$x]//g"|sort -r|head -1`;done
```
## Explanation
For each possible letter, filters it out only from input file resulting in something like this:
```
AAA
A
A
AAAA
A
AAAAAAAAAAAAAAAA
```
Then result is sorted and the longest line is selected. `echo -n` is there to remove newlines.
[Answer]
# Bash, 171 ~~159~~ 158, 138 with junk output
Requires lowercase-only input. Assumes that the file is called `_` (underscore). Maximum of 26 lines in the input file due to the annoying filenames which `split` creates (xaa, xab... xaz, ???).
In `bash`, `{a..z}` outputs `a b c d e f ...`.
```
touch {a..z}
split _ -1
for l in {a..z}
do for s in {a..z}
do grep -so $l xa$s>b$l
if [ `wc -l<b$l` -ge `wc -l<$l` ]
then mv b$l $l
fi
done
tr -d '\n'<$l
done
```
# Sample output
```
acdddeghillmmmoostvyy
```
# Explanation
```
touch {a..z}
```
Create files that we will be reading from later on so that bash doesn't complain that they don't exist. If you remove this line you will save 13 chars but get a lot of junk output.
```
split _ -1
```
Split the input file into sections, each storing 1 line. The files this command creates are named xaa, xab, xac and so on, I have no idea why.
```
for l in {a..z}
do for s in {a..z}
```
For each letter `$l` read through all lines stored in files `xa$s`.
```
do grep -so $l xa$s>b$l
```
Remove the `-s` switch to save 1 char and get a lot of junk output. It prevents `grep` from complaining about nonexistent files (will occur unless you have 26 lines of input).
This processes the file `xa$s`, removing anything but occurences of `$l`, and sending output to the file `b$l`. So "i love mommy" becomes "mmm" with new lines after each letter when `$l` is m.
```
if [ `wc -l<b$l` -ge `wc -l<$l` ]
```
If the number of lines in the file we just created is greater than or equal to (i.e. more letters since there is one letter per line) the number of lines in our highest result so far (stored in `$l`)...
```
then mv b$l $l
```
...store our new record in the file `$l`. At the end of this loop, when we have gone through all the lines, the file `$l` will store x lines each containing the letter `$l`, where x is the highest number of occurences of that letter in a single line.
```
fi
done
tr -d '\n'<$l
```
Output the contents of our file for that particular letter, removing new lines. If you don't want to remove the new lines, change the line with `tr` to `echo $l`, saving 6 chars.
```
done
```
[Answer]
## C#, 172 bytes
```
var x="";foreach(var i in File.ReadAllText(t).ToLower().Split('\r','\n'))foreach(var j in i)if(x.Count(c=>c==j)<i.Count(c=>c==j))x+=j;string.Concat(x.OrderBy(o=>o)).Trim();
```
[Answer]
# [05AB1E](https://github.com/Adriandmen/05AB1E/wiki/Commands), ~~12~~ ~~10~~ 8 [bytes](https://github.com/Adriandmen/05AB1E/wiki/Codepage)
```
Aεδ¢ày×?
```
-2 bytes thanks to *@ovs*, which also allowed -2 more bytes.
Input taken as a list of lowercase strings. ([Would require 1 additional byte if taking the input newline-delimited is mandatory.](https://tio.run/##yy9OTMpM/f@/xvHc1nNbDi06vKDy8HT7//8zUnNy8rkyFXLyy1IVkhNLYMyU/HQYMzc/N7eSC0yCBYoVUhJTUir/6@rm5evmJFZVAgA))
[Try it online.](https://tio.run/##yy9OTMpM/f/f8dzWc1sOLTq8oPLwdPv//6OVMlJzcvKVdJQyFXLyy1IVkhNLEJyU/HQEJzc/N7cSyAXTYKFihZTElJRKpdj/urp5@bo5iVWVAA)
**Explanation:**
```
A        # Push the lowercase alphabet
 ε       # For-each over the letters:
  δ¢     #  Count how many times this letter occurs in each string of the (implicit)
         #  input-list
    à    #  Pop and leave the maximum of these counts
     y×  #  Repeat the current letter that many times
       ? #  And print it (without newline)
```
[Answer]
# Python 2 - 129
Idea from @Tal
```
a,r=[0]*26,range(26)
for l in open('f'):a=[max(a[i],l.lower().count(chr(i+97)))for i in r]
print''.join(chr(i+97)*a[i]for i in r)
```
A couple more ways to do the same thing in the same number of characters:
```
a=[0]*26
b='(chr(i+97)))for i in range(26)'
exec'for l in open("f"):a=[max(a[i],l.lower().count'+b+']\nprint"".join(a[i]*('+b+')'
a=[0]*26
b='(chr(i+97)))for i in range(26))'
exec'for l in open("f"):a=list(max(a[i],l.lower().count'+b+'\nprint"".join(a[i]*('+b
```
This assumes the file is saved as f in an accessible directory. This program is directly runable, with no extra input necessary.
[Answer]
## Mathematica v10 - 110
It's not out yet, but reading [new documentation](https://reference.wolfram.com/language/guide/Associations.html) very carefully, I think this should work:
```
StringJoin@MapIndexed[#2~Table~{#1}&,Rest@Merge[Counts/@Characters@StringSplit[ToLowerCase@Input[],"\n"],Max]]
```
[Answer]
# Scala, 125 characters
```
val i=""::io.Source.stdin.getLines.toList.map(_.toLowerCase);println('a'to'z'map(c=>(""+c)*i.map(_.count(_==c)).max)mkString)
```
First I read the input, converting it into lower case and adding one empty line.
Then for each letter from `a` to `z` I repeat that letter maximum number of times it appears in any of the lines (that's why I need the empty line: `max` cannot be called on an enpty input). Then I just join the results and print to the output.
To read from a file, replace `stdin` with `fromFile("FILENAME")`, increasing the size of the code to 132 characters + file name length.
[Answer]
## Javascript, 261 chars
```
eval('s=prompt().toUpperCase().split("\\n");Z=[########0,0];H=Z.slice();s@r){h=Z.slice();r.split("")@c){if(c.match(/\\w/))h[c.charCodeAt(0)-65]++});H=H@V,i){return V>h[i]?V:h[i]})});s="";H@n,i){s+=Array(n+1).join(String.fromCharCode(i+97))});s'.replace(/@/g,".map(function(").replace(/#/g,"0,0,0,"))
```
Remove the `eval(...)` and execute to get the real code; this is (*somewhat*) compressed.
`s` multi-functions as the array of lines and as the outputted string, `h` contains the histogram of the letters per line and `H` contains the histogram with the maximum values up until now. It's case-insensitive, and just ignores anything but a-z and A-Z (I think... JS arrays are sometimes weird).
*Now correct :)*
[Answer]
# JavaScript (*ES5*) 141 bytes
Assuming variable `s` is the input string with no case-checking requirements and array output:
```
for(a in s=s[o=_='',y='split']('\n'))for(i=0;x=s[a][i++];)o+=x!=0&&(l=s[a][y](x).length-~-o[y](x).length)>0?Array(l).join(x):_;o[y](_).sort()
```
[Answer]
## PowerShell - 141
Reads text from a file named 'a'.
```
$x=@{}
gc a|%{[char[]]$_|group|%{$c=$_.name.tolower().trim()
$n=$_.count;$x[$c]=($n,$x[$c])[$n-lt$x[$c]]}}
($x.Keys|sort|%{$_*$x[$_]})-join""
```
[Answer]
## Groovy, 113/127 102/116 characters
Assuming the file is all in one case (102 chars):
```
t=new File('f').text;t.findAll('[A-Z]').unique().sort().each{c->print c*t.readLines()*.count(c).max()}
```
Assuming the file is in mixed case (116 chars):
```
t=new File('f').text.toUpperCase();t.findAll('[A-Z]').unique().sort().each{c->print c*t.readLines()*.count(c).max()}
```
Basically:
* `t=new File('f').text` To get the text of the file.
* `t.findAll('[A-Z]').unique().sort().each{c->` To get the unique characters, sort them, and iterate.
* `print c*t.readLines()*.count(c).max()` Get the max occurances in a single line and print the character that many times.
[Answer]
# Bash (mostly awk) - ~~172~~ ~~163~~ 157
```
awk -v FS="" '{delete l;for(i=1;i<=NF;i++)l[toupper($i)]++;for(i in l)o[i]=(o[i]>l[i]?o[i]:l[i])}END{for(i in o)for(j=0;j<o[i];j++)print i}'|sort|tr -d ' \n'
```
Text needs to be piped in to awk (or specified as a file).
### Example Input
```
Hello
I love cat
I love dog
I love mommy
Mommy loves daddy
```
### Example Output
```
ACDDDEGHILLMMMOOSTVYY
```
# PHP (probably could be better) - ~~174~~ 210
```
$o=array();foreach(explode("\n",$s) as $a){$l=array();$i=0;while($i<strlen($a)){$k=ucfirst($a[$i++]);if($k==' ')continue;$o[$k]=max($o[$k],++$l[$k]);}}ksort($o);foreach($o as $k=>$v)for($i=0;$i<$v;$i++)echo $k;
```
Assumes that the string is contained in the variable $s
### Example Input
```
Hello
I love cat
I love dog
I love mommy
Mommy loves daddy
```
### Example Output
```
ACDDDEGHILLMMMOOSTVYY
```
[Answer]
I realize this probably isn't the most efficient answer, but I wanted to try and solve the problem anyways. Here's my ObjC variation:
```
- (NSArray *) lettersNeededForString:(NSString *)sourceString {
    sourceString = [sourceString stringByReplacingOccurrencesOfString:@"\n" withString:@""];
    sourceString = [sourceString stringByReplacingOccurrencesOfString:@" " withString:@""];
    const char * sourceChars = sourceString.UTF8String;
    NSMutableArray * arr = [NSMutableArray new];
    for (int i = 0; i < sourceString.length; i++) {
        [arr addObject:[NSString stringWithFormat:@"%c", sourceChars[i]]];
    }
    return [arr sortedArrayUsingSelector:@selector(localizedCaseInsensitiveCompare:)];
}    
```
Then you can call it for whatever string:
```
NSArray * letters = [self lettersNeededForString:@"Hello\nI love cat\nI love dog\nI love mommy\nMommy loves daddy"];
NSLog(@"%@",letters);
```
I was thinking about applications with larger amounts of text and I'd rather not have to count my array. For this, I added to the method to get this:
```
- (NSDictionary *) numberOfLettersNeededFromString:(NSString *)sourceString {
    sourceString = [sourceString stringByReplacingOccurrencesOfString:@"\n" withString:@""];
    sourceString = [sourceString stringByReplacingOccurrencesOfString:@" " withString:@""];
    const char * sourceChars = sourceString.UTF8String;
    NSMutableArray * arr = [NSMutableArray new];
    for (int i = 0; i < sourceString.length; i++) {
        [arr addObject:[NSString stringWithFormat:@"%c", sourceChars[i]]];
    }
    static NSString * alphabet = @"abcdefghijklmnopqrstuvwxyz";
    NSMutableDictionary * masterDictionary = [NSMutableDictionary new];
    for (int i = 0; i < alphabet.length; i++) {
        NSString * alphabetLetter = [alphabet substringWithRange:NSMakeRange(i, 1)];
        NSIndexSet * indexes = [arr indexesOfObjectsPassingTest:^BOOL(id obj, NSUInteger idx, BOOL *stop) {
            if ([[(NSString *)obj lowercaseString] isEqualToString:alphabetLetter]) {
                return YES;
            }
            else {
                return NO;
            }
        }];
        masterDictionary[alphabetLetter] = @(indexes.count);
    }
    return masterDictionary;
}
```
Run like:
```
NSDictionary * lettersNeeded = [self numberOfLettersNeededFromString:@"Hello\nI love cat\nI love dog\nI love mommy\nMommy loves daddy"];
NSLog(@"%@", lettersNeeded);
```
Will give you:
> 
> {
>  a = 2;
>  b = 0;
>  c = 1;
>  d = 4;
>  e = 5;
>  f = 0;
>  g = 1;
>  h = 1;
>  i = 3;
>  j = 0;
>  k = 0;
>  l = 6;
>  m = 6;
>  n = 0;
>  o = 8;
>  p = 0;
>  q = 0;
>  r = 0;
>  s = 1;
>  t = 1;
>  u = 0;
>  v = 4;
>  w = 0;
>  x = 0;
>  y = 3;
>  z = 0;
>  }
> 
> 
> 
Which I think is better if I had a very large amount of text and I just needed to know how many of each letter I would need.
[Answer]
# K, 34
```
{`$a@<a:,/(.:a)#'!:a:|/#:''=:'0:x}
```
[Answer]
# [Haskell](https://www.haskell.org/), 41 bytes
```
f s=do c<-['a'..];maximum$filter(==c)<$>s
```
[Try it online!](https://tio.run/##NYwxCsMwDAD3vEKDIe1Qf6BxP1I6CMtORCQbKrc0r3dDIMtx3HAL2ppEes9ggSrE6fYccfT@dVf8sX7UZZaW3pcQ4nVyD@uKXAKXvWFsLnvhkqwv@6UODFK/CSK2U6nOp2pV3YaDRzAgJNr@ "Haskell – Try It Online") `f` takes a list of lines as input.
Add `main=interact$f.lines` to get a full program for a total of **63 bytes**.
**Explanation**
```
f s=                -- input s is the list of lines
 do c<-['a'..];     -- for each char c starting at c='a', concatenate the following strings
    filter(==c)<$>s -- for each line in s remove all characters except for the current c
   maximum$         -- take the longest of those strings
```
[Answer]
# Python 2, 154 bytes
```
import collections
c = collections.Counter()
for line in open("input.txt"):
    c |= collections.Counter(line.upper())
print "".join(sorted(c.elements()))
```
[Answer]
# C, 298 bytes
```
char c;
int j,n;
char C[26];
char D[26];
int main()
{
char a='a';
while((c=getchar())>=0)
{
c=tolower(c);
if(c>=a&&c<='z'){j=c-a;D[j]++;}
if(c=='\n'){
for(j=0;j<26;j++){
if(D[j]>C[j])
{C[j]=D[j];}
D[j]=0;
}
}
}
for(j=0;j<26;j++)
{
n=C[j];
while(n--)
{
putchar(a+j);
}
}
}
```
Array D holds tally of letters for each line, then the maximum count is copied to C.
Note: I put my answer yesterday but is now not listed, maybe I pressed delete instead of edit by mistake?
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