17. Letter Combinations of a Phone Number (backtracking)

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

/**
* Return an array of size *returnSize.
* Note: The returned array must be malloced, assume caller calls free().
*/
char* digit2Letter(char digit){
switch(digit){
case '2':
return "abc";
case '3':
return "def";
case '4':
return "ghi";
case '5':
return "jkl";
case '6':
return "mno";
case '7':
return "pqrs";
case '8':
return "tuv";
case '9':
return "wxyz";
default:
return "";
}
}

char** letterCombinations(char* digits, int* returnSize) {
char** returnArray = NULL;
if(*digits == '\0') return returnArray;

char* returnElem = malloc(sizeof(char)*sizeof(strlen(digits)));
returnArray = malloc(sizeof(char*)*1000);
backTracking(digits, returnArray, returnSize, returnElem, 0);

return returnArray;
}

void backTracking(char* digits, char** returnArray, int* returnSize, char* returnElem, int pElem){
if(*digits == '\0'){
(*returnSize)++;
char* elem = malloc(sizeof(char)*pElem);
memcpy(elem, returnElem, sizeof(char)*pElem);
elem[pElem] = '\0';
printf("elem[0] = %d, elem[1] = %d\n",elem[0],elem[1]);
returnArray[*returnSize-1] = elem;
return;
}

char* str = digit2Letter(*digits);
int len = strlen(str);
for(int i = 0; i < len; i++){
returnElem[pElem] = str[i];

backTracking(digits+1, returnArray, returnSize, returnElem, pElem+1 );
}
}