B题能用二分吗？

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;
typedef long long ll;
ll a[5050], b[5050];
ll ans = -1;
ll n, m, u;
ll f = 0;
void check(ll x)
{
	f = 0;
	ll l = 1, r = x;
	ll alla = 0, allb = 0;
	while (r <= n)
	{
		for (int i = l; i <= r; i++)
		{
			alla += a[i];
			allb += b[i];
		}
		if (alla <= m && allb <= u)
		{
			ans = max(ans, x);
			f = 1;
			return;
		}
		else l++, r++;
		alla = 0, allb = 0;
	}
}
signed main()
{
	cin >> n >> m >> u;
	for (int i = 1; i <= n; i++) cin >> a[i];
	for (int i = 1; i <= n; i++) cin >> b[i];
	ll cnt1 = 0, cnt2 = 0;
	for (int i = 1; i <= n; i++)
	{
		if (a[i] <= m) cnt1++;
		if (b[i] <= u) cnt2++;
	}
	ll cnt = max(cnt1, cnt2);
	ll l = 0, r = cnt + 2;
	while (l < r)
	{
		ll mid = l + r >> 1;
		check(mid);
		if (f == 0) r = mid;
		else l = mid + 1;
		f = 0;
	}
	if (l == r)
		check(l);
	cout << l;
}

感觉二分思路很清楚，但卡在87.5%