[LeetCode] Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

解题思路：

回溯法。通过递归进行回溯。代码如下。这里有个技巧，通过数组来记录每个数字可能的字母。另外，传递参数时，以字符指针指向当前扫描字符，传输效率更快。下面的代码在LeetCode的效率仅为2ms。

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        string letters[] = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        vector<string> result;
        char * c = (char*)digits.c_str();
        getLetters(result, "", c, letters);
        return result;
    }
    
    void getLetters(vector<string>& result, string s, char* c, string letters[]){
        if(*c == '\0'){
            if(s!="")
                result.push_back(s);
        }else{
            string letter = letters[*c - '0'];
            int len = letter.length();
            for(int i=0;i<len; i++){
                getLetters(result, s + letter[i], c+1, letters);
            }
        }
    }
};