[leetcode]Interleaving String

还是DFS啊，我是dp弱逼。。就是不懂DP，快哭了都

DFS+备忘录。。避免重复状态的计算

PS.

map,unordered_map还是有区别的

map要严格的弱序

unordered_map要求能hash

typedef string::const_iterator Iter;
map<pair<Iter,Iter> , bool> record;
class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        if(s1.size() + s2.size() != s3.size()) return false;
        record.clear();
        return dfs(s1.begin() , s1.end() , s2.begin() , s2.end() , s3.begin(), s3.end());
    }
private:
    bool dfs(Iter b1 , Iter e1 , Iter b2 , Iter e2 , Iter b3 , Iter e3) {
        if(b3 == e3) {
            return true;
        }
        auto tmp = record.find(make_pair(b1,b2));
        if(tmp != record.end()) return tmp->second;
        bool result = false;
        if(*b1 == *b3) result = dfs(b1+1,e1,b2,e2,b3+1,e3);
        if(result) {
            record.insert(make_pair(make_pair(b1,b2) , true));
            return true;
        }
        if(*b2 == *b3) result = dfs(b1,e1,b2+1,e2,b3+1,e3);
        if(result) {
            record.insert(make_pair(make_pair(b1,b2) , true));
            return true;
        }
        
        record.insert(make_pair(make_pair(b1,b2) , false));
        return false;
    }
};