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In today's LeetCode daily challenge we were tasked to sort an string by character frequency:

Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string. Return the sorted string. If there are multiple answers, return any of them.

I'm a beginner and I started learning C, I did my code and it apparently it was working, however when returning the answer in the code for the site, the output I was getting was "(NULL)".

My code is as follows:

char * frequencySort(char * s){
    int dictionary[56] = { 0 };  //array that stores character frequency
    int length = strlen(s);
    char ans[50000] = { '\0' };

   //for loop that read the string and stores character frequency
    for (int i = 0; i < length; i++) {
        int op = s[i]-65;
        dictionary[op]++;
    }

    //for loop that sorts the characters in order of frequency in the ans string
    for (int i = 0; i < length; i++) {
        int op = 0;
        for (int j = 0; j < 56; j++) {
            if (dictionary[j] > dictionary[op]) {
                op = j;
            }
        }
        ans[i] = op + 65;
        dictionary[op]--;
        while (dictionary[op] > 0) {
            ans[i + 1] = op + 65;
            i++;
            dictionary[op]--;
        }
    }
    return ans;
}

I tested it in VSCode and there I could use printf my ans string normally, and it would print the correct answer, like if the input was "tree", it would print "eert", but when returning the ans string to LeetCode, it returns (NULL), can anyone explain what I'm doing wrong? Sorry for any English mistakes, it's not my first language.

Rafael Carro Gaudim
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  • Does your compiler issue a warning regarding "returning address of local variable"? Listen to your compiler. That is illegal. The lifetime of `ans` ends as soon as you leave the function and you are not allowed to access that memory. – Gerhardh Dec 03 '22 at 15:13
  • I'm not an expert on this but looks like `ans` is allocated on the stack so when you return that address the behavior is undefined, you can have different results depending on what compiler and/or flags are used. – user700390 Dec 03 '22 at 15:13
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    Does this answer your question? [error: function returns address of local variable](https://stackoverflow.com/questions/12380758/error-function-returns-address-of-local-variable) – Gerhardh Dec 03 '22 at 15:14
  • Yeah, it seems that the problem was that I was using a local variable, and turning it global solves the problem, what a silly mistake, I usually tend to make most of the variables local because I was told it was better for management and readability, but I will keep in mind now of the returning these ones, thanks, if anyone wants to make an answer I'll gladly mark the post as answered. – Rafael Carro Gaudim Dec 03 '22 at 15:21
  • You might also look into dynamic memory allocation. Only use globals variables if really needed. – Gerhardh Dec 05 '22 at 15:29

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