[LeetCode] 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"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

问题描述：给定一个数字串，返回这个数字串可能代表的所有字符的组合。

数字与字符的组合如上，跟手机按键类似。

这题可以用递归的方式思考，例如"23"，2表示的字符串有"a", "b", "c"，然后在每个字符串后面添加3表示的每个字符。

class Solution {public:    vector<string> letterCombinations(string digits) {        // IMPORTANT: Please reset any member data you declared, as        // the same Solution instance will be reused for each test case.        map<char, string> phone;        phone.insert(make_pair('2', "abc"));        phone.insert(make_pair('3', "def"));        phone.insert(make_pair('4', "ghi"));        phone.insert(make_pair('5', "jkl"));        phone.insert(make_pair('6', "mno"));        phone.insert(make_pair('7', "pqrs"));        phone.insert(make_pair('8', "tuv"));        phone.insert(make_pair('9', "wxyz"));        phone.insert(make_pair('0', " "));        if(digits.size() == 0)            return vector<string>(1, "");        char ch = *(digits.end() - 1);        digits.erase(digits.end() - 1);        vector<string> svec = letterCombinations(digits);        vector<string>::size_type svec_size = svec.size();        vector<string>::size_type i = 0;        for(i = 0; i != svec_size; ++i) {            string str = svec[i];            string tmp = svec[i];            for(string::iterator siter = phone[ch].begin();                                 siter != phone[ch].end(); ++siter) {                tmp = str;                svec.push_back(tmp + *siter);            }        }        for(i = 0; i != svec_size; ++i) {            svec.erase(svec.begin());        }        return svec;    }};