51nod 1055 最长等差数列 (dp好题)

#include <bits/stdc++.h>
using namespace std;
const int maxn = 10010;
short dp[maxn][maxn];
int a[maxn];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  sort (a + 1, a + n + 1);
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= n; j++) {
      dp[i][j] = 2;
    }
  } 
  int ans = 2;
  for (int j = 1; j <= n; j++) {
    int i = j - 1, k = j + 1;
    while (i >= 1 && k <= n) {
      if (a[j] * 2 == a[i] + a[k]) {
        dp[j][k] = dp[i][j] + 1;
        ans = max(ans, (int)dp[j][k]);
        i--; k++;
      } else if (a[j] * 2 > a[i] + a[k]) {
        k++;
      } else {
        i--;
      }
    }
  }
  cout << ans << endl;
  return 0;
}