USACO 5.1 music theme

/*
ID: zlqest11
LANG: C++
TASK: theme
*/

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

const int N = 5010;

int n, num[N], dp[2][N];

int main()
{
    int ans = 1;
    freopen("theme.in", "r", stdin);
    freopen("theme.out", "w", stdout);
    scanf("%d", &n);
    for(int i = 1; i <= n; i++)  scanf("%d", num+i);
    for(int i = n; i > 1; i--){
        num[i] -= num[i-1];
    }
    int flag = 0;
    memset(dp, 0, sizeof(dp));
    for(int i = 1; i <= n; i++)
        dp[flag][i] = 1;
    for(int i = 2; i <= n; i++){
        flag ^= 1;
        for(int j = i+1; j <= n; j++){
            dp[flag][j] = 1;
            if(num[i] == num[j])
                dp[flag][j] = dp[flag^1][j-1]+1;
            if(dp[flag][j] > j-i)
                dp[flag][j] = j-i;
            ans = max(ans, dp[flag][j]);
        }
    }
    if(ans < 5)  ans = 0;
    printf("%d
", ans);
    return 0;
}