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传统手机透过数字键来打字，一个数字键通常代表了多个英文字母，例如2 代表a , b , c ，3 代表d , e , f 等，连续按多个键能产生英文字母的组合，此题为给定数字，输出所有可能的英文字母组合。

题目与范例如下

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 <= 4
digits[i] is a digit in the range ['2', '9'].
Accepted
861,326
Submissions
1,706,491

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解题方式为透过递归的方式，透过 for 回圈把每个数字可能的英文字母遍历出来，分别递归。填完字母后，补上结束字元 ' \0 ' 转存回要回传的空间内。

 

下方为我的代码

char map[10][5]={{},{},{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};
int reindex = 0;
char **re;
char *DPtr;
void Rnum( int index, char *tempstr){  // index use for input string index and tempstr index
	if(DPtr[index] == '\0'){
		tempstr[index] = '\0';
		re[reindex] = (char*)malloc(sizeof(char)*(strlen(DPtr)+1));
		strcpy(re[reindex],tempstr);
		reindex++;
	}
	else{
		for(int i = 0;i<strlen(map[DPtr[index]-'0']);i++){
			tempstr[index] = map[DPtr[index]-'0'][i];
			Rnum(index+1,tempstr);
		}
	}
}

char ** letterCombinations(char * digits, int* returnSize){
	DPtr = digits;
    reindex = 0;
	
    if(strlen(digits) != 0){
        (*returnSize) = 1;
        for(int i = 0 ; i < strlen(digits) ; i++ ){
            (*returnSize) *= strlen(map[digits[i]-'0']);
        }
    }
    else{
        (*returnSize) = 0;
    }
	re = (char**)malloc(sizeof(char*)*(*returnSize));
   
	char tempstr[5] = {0};
    if((*returnSize)>0)
	    Rnum( 0, tempstr);

    return re;

}

 

下方为时间与空间之消耗

Runtime: 0 ms, faster than 100 % of C online submissions for Letter Combinations of a Phone Number.

Memory Usage: 5.7 MB, less than 98.79 % of C online submissions for Letter Combinations of a Phone Number.