leetcode 17 phone numbers C language

Evaluation

topic

leetcode 17
given a numeric string containing only 2-9, it can return all letter combinations indicated.
Given digital map to letters as follows (the same telephone key). Note 1 does not correspond to any alphabet.

Example:
Input: "23"
outputs:. [ "Ad", " ae", "af", "bd", "be", "bf", "cd", "ce", "cf"]
Description:
Although the above answers are arranged in order according to the dictionary, but you can choose the order of the answer output.

void dfs(char *digits, char *str, int ind, char letter[][5], char **res, int *returnSize) {
    if (digits[ind] == '\0') {
        str[ind] = '\0';
        strcpy(res[*returnSize], str);
        (*returnSize)++;//谨记*的符号低于++，记得加括号
        return ;
    }
    int d = digits[ind] - '0', i = 0;
    while (letter[d][i] != '\0') {
        str[ind] = letter[d][i];
        dfs(digits, str, ind + 1, letter, res, returnSize);
        i++;
    }
}

char ** letterCombinations(char * digits, int* returnSize){
    *returnSize = 0;
    if (digits[0] == '\0') return digits;
    char letter[][5] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    int len = strlen(digits);
    int count = 1;
    for (int i = 0; i < len; i++) {
        count *= 4;
    }
    char **res = (char **)malloc(sizeof(char *) * count);
    for (int i = 0; i < count; i++) {
        res[i] = (char *)malloc(sizeof(char) * (len + 1));
        res[i][len] = '\0';
    }
    char *str = (char *)malloc(sizeof(char) * (len + 1));
    dfs(digits, str, 0, letter, res, returnSize);
    return res;
}