LeetCode-in-All
17. Letter Combinations of a Phone Number
Medium
Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.
A mapping of digit to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.
Example 1:
Input: digits = “23”
Output: [“ad”,”ae”,”af”,”bd”,”be”,”bf”,”cd”,”ce”,”cf”]
Example 2:
Input: digits = “”
Output: []
Example 3:
Input: digits = “2”
Output: [“a”,”b”,”c”]
Constraints:
0 <= digits.length <= 4digits[i]is a digit in the range['2', '9'].
Solution
/**
* Note: The returned array must be malloced, assume caller calls free().
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_COMBINATIONS 1000
void findCombinations(int start, const char *digits, const char **letters, char *curr, int depth, char ***ans, int *returnSize) {
if (depth == strlen(digits)) {
(*ans)[*returnSize] = strdup(curr);
(*returnSize)++;
return;
}
for (int i = start; i < strlen(digits); i++) {
int n = digits[i] - '0';
for (int j = 0; letters[n][j] != '\0'; j++) {
curr[depth] = letters[n][j];
findCombinations(i + 1, digits, letters, curr, depth + 1, ans, returnSize);
}
}
}
char **letterCombinations(const char *digits, int *returnSize) {
if (strlen(digits) == 0) {
*returnSize = 0;
return NULL;
}
const char *letters[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
char **ans = (char **)malloc(MAX_COMBINATIONS * sizeof(char *));
char *curr = (char *)malloc((strlen(digits) + 1) * sizeof(char));
curr[strlen(digits)] = '\0';
int returnCount = 0;
findCombinations(0, digits, letters, curr, 0, &ans, &returnCount);
free(curr);
*returnSize = returnCount;
return ans;
}