17. Letter Combinations of a Phone Number

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Example:

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

思考：按照如下图进行遍历。以输入为“234”为例。

 

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> res;
        if(digits.size()==0) return res;
        
        string map[] = {" ", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        string comb(digits.size(),'');
        recursion(digits, 1, comb, res,map);
        return res;
    }
    void recursion(string &digits, int len, string &comb, vector<string> &res, string map[]) {
        if(len==digits.size()+1) {
            res.push_back(comb);
            return;
        }
        
        string letters = map[digits[len-1] - '0'];
        for(int j=0; j<letters.size(); j++) {
            comb[len-1] = letters[j];
            recursion(digits, len+1, comb, res,map);
        }
    }
};