不能理解，为什么WA一个点？

#include <iostream>
#include <string>
#include <queue>
#include <limits.h>
#include <algorithm>
using namespace std;
const int N = 2e6 + 10;
typedef long long ll;
ll n , a[N] , q[N] , res;
int main()
{
	cin >> n;
	for (int i = 1 ; i <= n ; ++i) cin >> a[i];

	int hh = 0 , tt = -1 ;
	res = -0x3f3f3f3f;
	
	for (int i = 1 ; i <= n ; ++i)
	{
		if(hh <= tt) res = max(res , a[i] - a[q[hh]]);
		while(hh <= tt && a[q[tt]] >= a[i]) tt--;
		q[++tt] = i;
	}

	cout << res << endl;
	
	return 0;
}