[LeetCode] Letter Combinations of a Phone Number 回溯

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"].

    这题其实不难，注意好递归传入的参数便可以了。

#include <vector>
#include <string>
#include <iostream>
using namespace std;

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        ret.clear();
        if(digits.size()<=0)
            ret.push_back("");
        else
            help_f(0,digits,"");
        return ret;
    }
    vector<string> ret;
    vector<vector<char> >mp{{‘ ‘},
                            {},
                            {‘a‘,‘b‘,‘c‘},
                            {‘d‘,‘e‘,‘f‘},
                            {‘g‘,‘h‘,‘i‘},
                            {‘j‘,‘k‘,‘l‘},
                            {‘m‘,‘n‘,‘o‘},
                            {‘p‘,‘q‘,‘r‘,‘s‘},
                            {‘t‘,‘u‘,‘v‘},
                            {‘w‘,‘x‘,‘y‘,‘z‘}};
    void help_f(int nowIdx,string & digits,string curStr)
    {
        if(nowIdx==digits.size()){
            ret.push_back(curStr);
            return ;
        }
        int curNum = int(digits[nowIdx] - ‘0‘);
        for(int i =0;i<mp[curNum].size();i++)
            help_f(nowIdx+1,digits,curStr+mp[curNum][i]);
    }
};

int main()
{
    Solution sol;
    vector<string> ret=sol.letterCombinations("");
    for(int i=0;i<ret.size();i++)
        cout<<ret[i]<<endl;
    return 0;
}