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”].

#include "StdAfx.h"#include <string>#include <vector>#include <iostream>using namespace std;class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> res;        string trans[] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        size_t sz  = digits.size();        //判断digits的长度是否为0，如果为0，则直接返回，如果不加判断，res中会有个空串,即res的长度为1        if (0 == sz)        {            return res;        }        size_t n = 0;   //最开始处理第0个字符        string seq;        getLetterCombinations(digits,n,res,trans,seq);        return res;    }    void getLetterCombinations(string &digits, size_t n, vector<string> &res,string *trans,string &seq)    {        //如果处理到尾端        if (n == digits.size())        {            res.push_back(seq);            return ;        }        for (size_t i = 0;i<trans[digits[n]-'0'].size();++i)        {            seq.push_back(trans[digits[n]-'0'][i]);            getLetterCombinations(digits,n+1,res,trans,seq);            //回溯            seq.resize(seq.size()-1);        }    }};int main(){    Solution s;    vector<string> vs;    vs = s.letterCombinations("");    cout<<vs.size()<<endl;    for (vector<string>::iterator it=vs.begin();it!=vs.end();++it)    {        cout<<*it<<'\t';    }    return 0;}