【Leetcode长征系列】Letter Combinations of a Phone Number

原题：

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.

思路：涉及到排列组合问题，使用递归专门处理组合问题。一开始老是runtime error，主要问题出在string char以及const char的转换上，比较烦人。思路比较简单的。

代码：

class Solution {public:    vector<string> letterCombinations(string digits) {        vector<string> res, tmp;        if(digits.size()==0) {            string str = "";            res.push_back(str);            return res;        }        for (int i = 0; i<digits.size(); i++){            string accord = digitString(digits[i]);            tmp.push_back(accord);        }        return makecombine(tmp.begin(),tmp.end());    }        vector<string> makecombine(vector<string>::iterator first, vector<string>::iterator last){        vector<string> res,tmp;        if(first==last) return res;        tmp = makecombine(first+1,last);        for(string::iterator it = (*first).begin(); it!=(*first).end(); it++){            if(tmp.empty())  {                string t = "";                t = t + *it;                res.push_back(t);            }            else {                for(int j = 0; j<tmp.size(); j++){                    string t = "";                    t = *it;                    t = t+tmp[j];                    res.push_back(t);                }            }        }        return res;    }        string digitString(char a){        string str= "";        if (a-'0'==0) str =' ';        else if(a -'0'== 1) ;        else if(a -'0'== 2) str = "abc";        else if(a -'0'== 3) str = "def";        else if(a -'0'== 4) str = "ghi";        else if(a -'0'== 5) str = "jkl";        else if(a -'0'== 6) str = "mno";        else if(a -'0'== 7) str = "pqrs";        else if(a -'0'== 8) str = "tuv";        else if(a -'0'== 9) str = "wxyz";        return str;    }};

AC来的太突然受宠若惊- 。-