poj 1868 Antiarithmetic? 推断序列中是否有等差子序列

Posted 2020-10-03 slgkaifa

#include<iostream>
using namespace std;
const int maxN=10024;
int n,vis[maxN],a[maxN];

bool find()
{
	memset(vis,-1,sizeof(vis));
	for(int i=0;i<n;++i){
		for(int j=0;j<i;++j){
			int t=2*a[i]-a[j];
			if(t>=0&&t<n&&vis[t]==-1)
				return true;
		}
		vis[a[i]]=1;
	}	
	return false;
}

int main()
{
	while(scanf("%d",&n)==1&&n){
		scanf("%*c");
		for(int i=0;i<n;++i)
			scanf("%d",&a[i]);
		if(find())
			puts("no");
		else
			puts("yes");
	}
	return 0;	
}