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

#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");elseputs("yes");}return 0;}

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