[leetcode]Distinct Subsequences

5555,又是DP。。。然后我又是DFS+record

其实关键是理解题意，写出DFS代码就好了吧。。。。。

map<pair<int,int>,int> record;
class Solution {
public:
    int numDistinct(string S, string T) {
        record.clear();
        return vis(S, 0 , S.size() , T , 0 , T.size());
    }
private:
    int vis(string s , int s1 , int e1 , string t , int s2 , int e2) {
        if(s1 == e1) {
            return s2 == e2 ? 1 : 0;
        }
        if(s2 == e2) return 1;
        auto tmp = record.find(make_pair(s1,s2));
        if(tmp != record.end()) return tmp->second;
        
        int cnt = 0;
        for(int i = s1 ; i < e1 ; ++i) {
            if(s[i] == t[s2]) {
                cnt += vis(s , i + 1 , e1 , t , s2 + 1 , e2);
            }
        }
        record.insert(make_pair(make_pair(s1,s2) , cnt));
        return cnt;
    }
};