poj 2362

回溯加剪枝

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <set>
#include <sstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#define maxn 1000010
#define INF 0x7fffffff
#define inf 10000000
#define ull unsigned long long
#define ll long long
using namespace std;

int a[30], n;
bool vis[30];

bool dfs(int cur, int now, int aim, int k)
{
    if(cur == 3) return true;
    for(int i = k; i < n; ++ i)
    {
        if(!vis[i] && now > a[i])
        {
            vis[i] = 1;
            if(dfs(cur, now-a[i], aim, i+1))
                return true;
            vis[i] = 0;
        }
        else if(!vis[i] && now == a[i])
        {
            vis[i] = 1;
            if(dfs(cur+1, aim, aim, 0))
                return true;
            vis[i] = 0;
        }
    }
    return false;
}
int main()
{
    int t, sum;
    scanf("%d", &t);
    while(t--)
    {
        sum = 0;
        int _max = 0;
        memset(vis, 0, sizeof(vis));
        scanf("%d", &n);
        for(int i = 0; i < n; ++ i)
        {
            scanf("%d", &a[i]);
            sum += a[i];
            _max = max(_max, a[i]);
        }
        if(sum % 4 || _max > sum/4)
            puts("no");
        else
        {
            if(dfs(0, sum/4, sum/4, 0)) puts("yes");
            else puts("no");
        }
    }
    return 0;
}