leetcode 17:Letter Combinations of a Phone Number（15-10-5）

1.Problem

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.

2.Solution

class Solution {public:vector<string> letterCombinations(string digits) {    vector<string> res;    if(digits[0]=='\0') {return res;}    string charmap[10] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};    res.push_back("");    for (int i = 0; i < digits.size(); i++)    {        vector<string> tempres;        string chars = charmap[digits[i] - '0'];        for (int c = 0; c < chars.size();c++)            for (int j = 0; j < res.size();j++)                tempres.push_back(res[j]+chars[c]);        res = tempres;    }    return res;}};