取对子游戏：http://www.nfls.com.cn:20035/contest/1878/problem/5

http://www.nfls.com.cn:20035/submission/781868

#include<bits/stdc++.h>
using namespace std;
int N, ct[45], b[25], ans, a[45][5];
void dfs(int t, int sum){//sum几张非对子 
	if(sum >= 5) return;
	if(t == 4*N+1 || sum < 5){
		ans = max(ans, (t-sum)/2);//最多的对数	
	}
	for(int i = 1; i <= 4; i++){
		if(ct[i] + 1 <= N){
			ct[i]++;
			int x = a[ct[i]][i]; 
			if(!b[x]){
				b[x] = 1;
				dfs(t+1, sum+1);
				b[x] = 0;		
			} 
			else if(b[x] % 2 == 1){
				b[x] = 0;
				dfs(t+1, sum-1);
				b[x] = 1;
			}
			ct[i]--;//第i列-- 
		}
	}
}
int main(){
	cin>>N;
	for(int i = 1; i <= N; i++){
		for(int j = 1; j <= 4; j++){
			scanf("%d",&a[i][j]);
		}
	}
	dfs(1,0); 
	cout<<ans<<endl;
	return 0;
}


right:
#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
int N, ct[45], b[25], ans, a[45][5], f[45][45][45][45];
void dfs(int t, int sum){//每层取一个数字,sum几张非对子
	if(sum >= 5) return;
    if(f[ct[1]][ct[2]][ct[3]][ct[4]]) return;
    f[ct[1]][ct[2]][ct[3]][ct[4]] = 1;
	ans = max(ans, (t-sum)/2);//最多的对数
	for(int i = 1; i <= 4; i++){
		if(ct[i] + 1 <= N){
			ct[i]++;
			int x = a[ct[i]][i];
			if(!b[x]){
				b[x] = 1;
				dfs(t+1, sum+1);
				b[x] = 0;
			}
			else if(b[x] % 2 == 1){
				b[x] = 0;
				dfs(t+1, sum-1);
				b[x] = 1;
			}
			ct[i]--;//第i列--
		}
	}
}
int main(){
	cin>>N;
	for(int i = 1; i <= N; i++){
		for(int j = 1; j <= 4; j++){
			scanf("%d",&a[i][j]);
		}
	}
	dfs(1,0);
	cout<<ans<<endl;
	return 0;
}