B. Solitaire

https://codeforces.com/problemset/problem/208/B

思路

dfs+回溯 对两种操作情况进行 地毯式搜索，

找到可能的一种方法。

Note：

需要使用cache对已经搜索过的路径的结果，做记录， 减少重复计算。参考：

https://blog.csdn.net/soyamilk233/article/details/78057093

Code

https://codeforces.com/problemset/submission/208/192943593

#include <bits/stdc++.h>
using namespace std;


map<string, bool> cache;
map<string, bool> hit;


void print_deck(vector<pair<char,char> >& v){
    cout << "------------------- deck:" << endl;

    for(int i=0; i<v.size(); i++){
        cout << v[i].first << v[i].second << " ";
    }
    cout << endl;
}

bool match(pair<char,char> a, pair<char,char> b){
    return a.first==b.first||a.second==b.second;
}

bool dfs(vector<pair<char,char> >& v,
            int n){
    string path;
    for(int i=0; i<n; i++){
        path += v[i].first + v[i].second;
    }

    if (hit[path]){
        return cache[path];
    }

    if (n == 1){
        hit[path] = true;
        cache[path] = true;
        return true;
     }

    pair<char, char> top = v[n-1];

    if(n>=2){
        pair<char, char> prev = v[n-2];
        
        if (match(top,prev)){
            v[n-2] = top;
            
            if (dfs(v, n-1)){
                cache[path] = true;
                hit[path] = true;
                
                v[n-2] = prev;
                
                return cache[path];
            }
            
            v[n-2] = prev;
        }
    }
    
    if(n >= 4){
        pair<char, char> prev = v[n-4];
        
        if (match(top,prev)){
            v[n-4] = top;
            
            if (dfs(v, n-1)){
                cache[path] = true;
                hit[path] = true;
                
                v[n-4] = prev;

                return cache[path];
            }
            
            v[n-4] = prev;
        }
    }
    
    cache[path] = false;
    hit[path] = true;

    return cache[path];
}

int main(){
    int n;
    cin>>n;

    vector<pair<char,char> >v(n);

    for(int i=0;i<n;i++){
        cin>>v[i].second>>v[i].first;
    }
    
    cout<<(dfs(v,n)?"YES":"NO")<<endl;
}