Extremely large tests are very hard to debug, because we are not able to understand the input anymore. The only choice that is left is to go back and look at the code, to potentially find any flaws. I find it quite hard to do this, as sometimes it is very hard to find the bug just looking at the code, especially when the code is confusing.

I was about to finish a problem called "Hotels" from POI, but I got 99%. There was only one case wrong and it was one of the larger cases (N = 5000). Now I'm stuck, because I couldn't find anything wrong with the code, even though it is relatively short. I used an approach similar to this. Would you guys be so kind to point out if I did anything wrong? Also, how do you guys deal with these situations?

#include <bits/stdc++.h>
using namespace std;

int n;
long long ans, tmp[5001], sum[5001], p[5001];
vector<int> g[5001];

void dfs(int u, int p, int d) {
	tmp[d]++;
	for(int i = 0, v = g[u][i]; i < g[u].size(); i++, v = g[u][i]) if(v != p) dfs(v, u, d+1);
}

int main() {
	scanf("%d",&n);
	for(int i = 1,u,v; i < n; i++) {
		scanf("%d%d",&u,&v);
		g[u].push_back(v);
		g[v].push_back(u);
	}
	for(int i = 1; i <= n; i++) {
		fill(sum+1, sum+n+1, 0);
		fill(p+1, p+n+1, 0);
		for(int j = 0; j < g[i].size(); j++) {
			fill(tmp+1, tmp+n+1, 0);
			dfs(g[i][j], i, 1);
			for(int k = 1; k <= n; k++) {
				ans += tmp[k]*p[k];
				p[k] += tmp[k]*sum[k]; // distributive property
				sum[k] += tmp[k];
			}
		}
	}
	printf("%lld", ans);
}