代码：

#include<bits/stdc++.h>
using namespace std;
int n,a[262145],f[262145][59],ans;
int main(){
	scanf("%d",&n);
	for(int i = 1;i <= n;i ++){
		scanf("%d",&a[i]);
		f[i][a[i]] = i;
		ans = max(ans,a[i]);
	} 
	for(int num = 2;num <= 58;num ++){
		for(int i = 1;i < n;i ++){
			if(f[i][num - 1] && f[f[i][num - 1] + 1][num - 1]){
				f[i][num] = f[f[i][num - 1] + 1][num - 1];
				ans = max(ans,num);
			}
		}
	}
	printf("%d",ans);
	return 0;
}

左闭右闭，但是有特判，应该不是这个问题吧
（Wrong Answer.wrong answer On line 1 column 2, read 8, expected 7.）