题目：

题解：

char phoneMap[11][5] = {"\0", "\0", "abc\0", "def\0", "ghi\0", "jkl\0", "mno\0", "pqrs\0", "tuv\0", "wxyz\0"};char* digits_tmp;
int digits_size;char** combinations;
int combinations_size;char* combination;
int combination_size;void backtrack(int index) {if (index == digits_size) {char* tmp = malloc(sizeof(char) * (combination_size + 1));memcpy(tmp, combination, sizeof(char) * (combination_size + 1));combinations[combinations_size++] = tmp;} else {char digit = digits_tmp[index];char* letters = phoneMap[digit - '0'];int len = strlen(letters);for (int i = 0; i < len; i++) {combination[combination_size++] = letters[i];combination[combination_size] = 0;backtrack(index + 1);combination[--combination_size] = 0;}}
}char** letterCombinations(char* digits, int* returnSize) {combinations_size = combination_size = 0;digits_tmp = digits;digits_size = strlen(digits);if (digits_size == 0) {*returnSize = 0;return combinations;}int num = 1;for (int i = 0; i < digits_size; i++) num *= 4;combinations = malloc(sizeof(char*) * num);combination = malloc(sizeof(char*) * digits_size);backtrack(0);*returnSize = combinations_size;return combinations;
}