LeetCode--Letter Combinations of a Phone Number

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题目描述：

Given a string containing digits from 2-9 inclusive, 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. Note that 1 does not map to any letters.

Letter-Combinations-of-a-Phone-Number

note：

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

样例：

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Input: "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

题意：

给定只包含 2 - 9 之间的一个字符串代表一个数字，返回这个数字所有的组合

思路：

就是根据 2 - 9 所对应的字符进行一个标记，然后直接 dfs 遍历所有的可能的结果。

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class Solution {
public:
    vector<string> letterCombinations(string digits) {
        map<string, string> mp;
        mp["2"] = "abc";
        mp["3"] = "def";
        mp["4"] = "ghi";
        mp["5"] = "jkl";
        mp["6"] = "mno";
        mp["7"] = "qprs";
        mp["8"] = "tuv";
        mp["9"] = "wxyz";
        vector<string> res;
        if(digits.empty() || digits.size() == 0) return res;
        string tmp;
        dfs(res, digits, 0, tmp, mp);
        return res;
    }
    
    void dfs(vector<string> &res, string digits, int cur, string &tmp, map<string, string> mp){
        if(cur == digits.size()){
            res.push_back(tmp);
            return;
        }
        string index;
        index += digits[cur];
        for(int i = 0; i < mp[index].size(); ++i){
            tmp.push_back(mp[index][i]);
            dfs(res, digits, cur + 1, tmp, mp);
            tmp.pop_back();
        }
    }
};

Runtime：8ms Memory：10.3MB