LeetCode OJ-- Letter Combinations of a Phone Number ***

https://oj.leetcode.com/problems/letter-combinations-of-a-phone-number/

使用递归，深搜，使用 map 保存已经处理过的结果

class Solution {
public:
    unordered_map<string, vector<string> > memory;
    
    vector<string> letterCombinations(string digits) {
        vector<string> ans;
        
        if(digits.size() == 0)
        {
            ans.push_back("");
            return ans;
        }
        
        memory.clear();
        
        // initialize
        vector<string> piece;
        piece.push_back("a");
        piece.push_back("b");
        piece.push_back("c");
        memory.insert(make_pair("2",piece));
        
        vector<string> piece3;
        piece3.push_back("d");
        piece3.push_back("e");
        piece3.push_back("f");
        memory.insert(make_pair("3",piece3));
        
        vector<string> piece4;
        piece4.push_back("g");
        piece4.push_back("h");
        piece4.push_back("i");
        memory.insert(make_pair("4",piece4));
        
        vector<string> piece5;
        piece5.push_back("j");
        piece5.push_back("k");
        piece5.push_back("l");
        memory.insert(make_pair("5",piece5));
        
        vector<string> piece6;
        piece6.push_back("m");
        piece6.push_back("n");
        piece6.push_back("o");
        memory.insert(make_pair("6",piece6));
        
        vector<string> piece7;
        piece7.push_back("p");
        piece7.push_back("q");
        piece7.push_back("r");
        piece7.push_back("s");
        memory.insert(make_pair("7",piece7));
        
        vector<string> piece8;
        piece8.push_back("t");
        piece8.push_back("u");
        piece8.push_back("v");
        memory.insert(make_pair("8",piece8));
        
        vector<string> piece9;
        piece9.push_back("w");
        piece9.push_back("x");
        piece9.push_back("y");
        piece9.push_back("z");
        memory.insert(make_pair("9",piece9));
        
        subLetter(digits, ans);
        return ans;
    }
    
    void subLetter(string &digits, vector<string> &ans)
    {
        // already in 
        if(memory.find(digits) != memory.end())
        {
            ans = memory[digits];
            return;
        }
        
        // split digits
        int mid = digits.size()/2;
        string former = digits.substr(0,mid);
        string latter = digits.substr(mid,digits.size() - mid);
        
        vector<string> strsFormer;
        vector<string> strsLatter;
        if(memory.find(former) != memory.end())
            strsFormer = memory[former];
        else
            subLetter(former, strsFormer);
        
        if(memory.find(latter) != memory.end())
            strsLatter = memory[latter];
        else
            subLetter(latter,strsLatter);
        
        for(int i = 0; i < strsFormer.size(); i++)
            for(int j = 0; j < strsLatter.size(); j++)
            {
                string temp = strsFormer[i] + strsLatter[j];
                ans.push_back(temp);
            }
        
        memory.insert(make_pair(digits,ans));
        return;
    }
    
};