LeetCode17. 电话号码的组合

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例:

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

解题思路：

通过输入的digits、找到对应的字母，然后将目前的字母和已有的字母进行组合。

class Solution {
public:
    vector<string> letterCombinations(string digits) {
        vector<string> s1,s2;
        map<char,vector<string>> mp;
        mp['2']={"a","b","c"};
        mp['3']={"d","e","f"};
        mp['4']={"g","h","i"};
        mp['5']={"j","k","l"};
        mp['6']={"m","n","o"};
        mp['7']={"p","q","r","s"};
        mp['8']={"t","u","v"};
        mp['9']={"w","x","y","z"};
        for(auto &ch:digits){
            int size=s1.size();
            auto &v=mp[ch];
            for(auto &s:v){
                for(int i=0;i<size;i++){
                    string tmp=s1[i]+s;
                    s2.push_back(tmp);
                }
                if(size==0) s2.push_back(s);
            }
            swap(s1,s2);
            s2.clear();
        }
        return s1;
    }
};

https://blog.csdn.net/lv1224/article/details/79767427