[leetcode] 17. Letter Combinations of a Phone Number 解题报告

题目链接：https://leetcode.com/problems/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.

思路：建一张表，简单的深搜。每次取那个数字的字母表的一个值。

代码如下：

class Solution {
public:
    void DFS(string str, int k, string tem)
    {
        if(k >= str.size()) result.push_back(tem);
        if(!(str[k]>'1' && str[k]<= '9')) return;
        for(auto val: hash[str[k]-'0']) 
            DFS(str, k+1, tem+val);
    }
    vector<string> letterCombinations(string digits) {
        if(digits.size() ==0) return {};
        hash[2] = {'a', 'b', 'c'};
        hash[3] = {'d', 'e', 'f'};
        hash[4] = {'g', 'h', 'i'};
        hash[5] = {'j', 'k', 'l'};
        hash[6] = {'m', 'n', 'o'};
        hash[7] = {'p', 'q', 'r', 's'};
        hash[8] = {'t', 'u', 'v'};
        hash[9] = {'w', 'x', 'y', 'z'};
        DFS(digits, 0, "");
        return result;
    }
private:
    vector<string> result;
    unordered_map<int, set<char>> hash;
};