HDU 1518 Square

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dfs+剪枝！

AC代码如下：

///dfs+剪枝  62MS 300K#include<iostream>#include<cstring>#include<algorithm>#include<cstdio>using namespace std;int n,m;int a[50];int vis[50];bool cmp(int a,int b){    return a>b;}int dfs(int v,int cur,int bg){    if(cur==4)        return 1;    int i;    for(i=bg;i<n;i++)    {        int flag=0;        if(!vis[i]&&a[i]+v==m)        {            flag=1;            vis[i]=1;            if(dfs(0,cur+1,0))                return 1;            vis[i]=0;            return 0;        }        if(!vis[i]&&a[i]+v<m)        {            flag=1;            vis[i]=1;            if(dfs(a[i]+v,cur,i+1))                return 1;            vis[i]=0;            if(v==0)                return 0;        }        while(flag&&i+1<n&&a[i+1]==a[i])            i++;    }    return 0;}int main(){    int t;    int i;    scanf("%d",&t);    while(t--)    {        int sum=0;        scanf("%d",&n);        for(i=0;i<n;i++)        {            scanf("%d",&a[i]);            sum+=a[i];        }        sort(a,a+n,cmp);        if(sum%4!=0||a[0]>sum/4)        {            printf("no\n");            continue;        }        m=sum/4;        memset(vis,0,sizeof vis);        if(dfs(0,0,0))            printf("yes\n");        else printf("no\n");    }    return 0;}

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