Leetcode Letter Combinations of a Phone Number

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给出手机上的字符按键顺序，求所有可能的按出的字符串。简单dfs搜索，0ms

const char* map[] = {"abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};void dfs(char* digits, char* pre, char** res, int* returnSize) {    if (!digits[0]) {        res[*returnSize] = malloc(strlen(pre) + 1);        strcpy(res[*returnSize], pre);        (*returnSize)++;        return;    }    int i;    char* mapstr = map[digits[0] - '2'];    for (i = 0 ; i < strlen(mapstr) ; i++) {        int oldpre = strlen(pre);        pre[oldpre] = mapstr[i];        pre[oldpre+1] = 0;        dfs(digits+1, pre, res, returnSize);        pre[oldpre] = 0;    }}char** letterCombinations(char* digits, int* returnSize) {    int i, j;    *returnSize = 0;    if (digits[0] == 0) return NULL;    for (i = 0 ; i < strlen(digits) ; i++)        if (!strchr("23456789", digits[i]))            return NULL;    char** res = malloc(sizeof(char*) * (27 * 27 * 16));    char* pre = malloc(strlen(digits) + 1);    pre[0] = 0;    dfs(digits, pre, res, returnSize);    free(pre);    return res;}

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