Common Characters

May 14, 2019

Today’s exercise comes from a coding challenge site:

Given a list of words containing only lower-case letters, return a list of characters that appear in all the words, with multiplicity. For instance, given the words BELLA, LABEL and ROLLER, the characters E, L and L appear in all three words.

Your task is to return a list of characters that appear in all words of an input list. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.


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3 Responses to “Common Characters”

  1. Globules said

    A Haskell version.

    import Data.List (foldr1, intersect)
    common :: Eq a => [[a]] -> [a]
    common []  = []
    common xss = foldr1 intersect xss
    main :: IO ()
    main = do
      print $ common ([] :: [String])
      print $ common ["BELLA"]
      print $ common ["BELLA", "LABEL"]
      print $ common ["BELLA", "LABEL", "ROLLER"]
    $ ./commchar
  2. Paul said

    In Python.

    from collections import Counter
    from functools import reduce
    from operator import and_
    def common(listofwords):
        return list(reduce(and_, (Counter(w) for w in listofwords)).elements())
    print(common(["BELLA", "LABEL"]))  # -> ['B', 'E', 'L', 'L', 'A']
    print(common(["BELLA", "LABEL", "ROLLER"]))  # -> ['E', 'L', 'L']
  3. Daniel said

    Here’s a solution in C.

    #include <stdio.h>
    #include <stdlib.h>
    // Returns the result in s1
    void common(char* s1, char* s2) {
      int a1[26] = {0};
      int a2[26] = {0};
      char* _s1 = s1;
      for (; *s1; ++s1) {
        ++(a1[*s1 - 'a']);
      for (; *s2; ++s2) {
        ++(a2[*s2 - 'a']);
      int pos = 0;
      for (int i = 0; i < 26; ++i) {
        int count = a1[i] < a2[i] ? a1[i] : a2[i];
        for (int j = 0; j < count; ++j) {
          _s1[pos++] = 'a' + i;
      _s1[pos] = '\0';
    int main(int argc, char* argv[]) {
      if (argc == 1) return EXIT_FAILURE;
      for (int i = 1; i < argc; ++i) {
        common(argv[1], argv[i]);
      printf("%s\n", argv[1]);
      return EXIT_SUCCESS;

    Example Usage:

    $ ./a.out bella label roller

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