LeetCode: Letter Combinations of a Phone Number

思路：递归求解，为求当前的str表示的字符串，可以先求出str[1]开始的子串数字能够表示的字符串。然后再将str[0]指示的字符依次贴到这些字符串中，得到最终解。

code:

class Solution {public:    vector<string> letterCombinations(string digits) {        string word[] = {" ","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};        vector<string> ret;        int digitLen = digits.length();        if(digitLen>0){            int len = word[digits[0] -'0'].length();            string subDigits = digits.substr(1,digitLen-1);            vector<string> subRet;            subRet = letterCombinations(subDigits);            int subLen = subRet.size();            for(int i=0;i<len;i++){                for(int j=0;j<subLen;j++){                    string str = word[digits[0] -'0'][i] + subRet[j];                    ret.push_back(str);                }            }        }        else            ret.push_back("");        return ret;    }};