17. 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.

class Solution {
public:
    vector<string> letterCombinations(string digits) {
    vector<string> res;
    string ss[10] = {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    if(digits.size() == 0 ) return res;
    res.push_back("");
    for (int i = 0; i < digits.size(); i++)
    {
        vector<string> tempres;
        string chars = ss[digits[i] - '0'];     //当第i个字符digits[i] = “3”，那么digits[i] - '0'把它转化为数字3，则此时chars = “def”
        for (int c = 0; c < chars.size();c++)    //遍历chars里面的字符，再分别与res里面已有的字串结合
            for (int j = 0; j < res.size();j++)
                tempres.push_back(res[j]+chars[c]);
        res = tempres;
    }
    return res;
}
};

//相当于输入“342”。第一次取“3”，res={"d","e","f"},
// 第二次取“4”，对应“ghi”元素分别与res结合再放入临时变量tempers中，
//接着=tempers，即res={“dg”，“dh”，“di”，“eg”，“eh”，“ei”，“fg”，“fh”，“fi”}
//第三次取“2”，对应“abc”元素分别与res结合再放入临时变量tempers中，结果很长就不写了。。

 https://leetcode.com/problems/letter-combinations-of-a-phone-number/discuss/ discussion区有很多好的思路，感兴趣可以看看