Letter Combinations of a Phone Number

题目

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

排列组合的递归方法

class Solution {public:    vector<string> letterCombinations(string digits) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int len = digits.length();        vector<string> result;        string str = "";          myLettercomb(result, str, digits, 0);        return result;    }        void myLettercomb(vector<string> &result, string & str, string &digits, int cur) {        const static string key_maps[10]={"0","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        if(cur==digits.length()) {            result.push_back(str);            return ;        }        int num = digits[cur]-'0';        for(int i=0; i<key_maps[num].length(); i++) {            str = str+key_maps[num][i];            myLettercomb(result, str, digits, cur+1);            str.erase(str.length()-1);        }            }        };