51nod1055 最长等差数列

 1 #include<cstdio>
 2 #include<algorithm>
 3 #define N 10010
 4 #define rg register
 5 using namespace std;
 6 short n,f[N][N],ans;
 7 int a[N];
 8 inline int read(){
 9     int k=0,f=1; char c=getchar();
10     while(c<‘0‘||c>‘9‘)c==‘-‘&&(f=-1),c=getchar();
11     while(‘0‘<=c&&c<=‘9‘)k=k*10+c-‘0‘,c=getchar();
12     return k*f;
13 }
14 inline short max(short x,short y){return x>y?x:y;}
15 int main(){
16     n=read();
17     for(rg int i=1;i<=n;i++) a[i]=read();
18     sort(a+1,a+1+n);
19     for(rg int i=0;i<=n;i++)
20     for(rg int j=0;j<=n;j++) f[i][j]=2;
21     for(rg int i=1;i<=n-1;i++){
22         int pre=i-1;
23         for(rg int j=i+1;j<=n;j++){
24             while(pre&&a[j]-a[i]>a[i]-a[pre]) pre--;
25             if(!pre) break;
26             if(pre&&a[j]-a[i]==a[i]-a[pre]) f[j][i]=max(f[j][i],f[i][pre]+1);
27             ans=max(ans,f[j][i]);
28         }
29     }
30     printf("%d\n",ans);
31     return 0;
32 }