以下是代码部分

以下是代码部分

#include<iostream>
using namespace std;
const int maxn = 5005;
int head[maxn];
int tail[maxn * 2];
int next[maxn * 2];
int son[maxn];
int m = 1;
int d[maxn];
int F[maxn];
int G[maxn];
int* f[maxn];
int* g[maxn];

void link(int a, int b) {
  m++;
  tail[m] = b;
  next[m] = head[a];
  head[a] = m;
}

void dfs1(int u, int fa) {
	d[u] = 1;
	for (int e = head[u]; e != 0; e = next[e]) {
		int v = tail[e];
		if (v != fa) {
			dfs1(v, u);
			if (d[u] < d[v] + 1) {
				son[u] = v;
				d[u] = d[v] + 1;
			}
		}
	}
}

long long ans = 0;

void dfs(int u, int fa, int pt) {
	f[u] = F + pt;
	g[u] = G + pt;
	int rr = pt;
	pt += d[son[u]] + 1;
	f[u][0] = 1; 
	for (int e = head[u]; e != 0; e = next[e]) {
		int v = tail[e];
		if (v == son[u]) continue;
		if (v != fa) {
			dfs(v, u, pt);
			pt += d[v];
		}
	}
	if (son[u] > 0)
		dfs(son[u], u, rr + 1);
	for (int i = 0; i + 1 < d[son[u]]; ++i) {
		g[u][i] = g[son[u]][i + 1];
	}
	for (int e = head[u]; e != 0; e = next[e]) {
		int v = tail[e];
		if (v != fa && v != son[u]) {
			for (int i = 0; i < d[v]; ++i) {
				ans += (long long)g[u][i+1] * f[v][i];
			}
			for (int i = 0; i < d[v]; ++i){
				ans += (long long)f[u][i] * g[v][i+1];
			}
			for (int i = 0; i < d[v]; ++i) {
				g[u][i+1] += f[v][i] * f[u][i+1];
			}
			for (int i = 0; i + 1 < d[v]; ++i) {
				g[u][i] += g[v][i+1];
			}
			for (int i = 0; i < d[v]; ++i) {
				f[u][i+1] += f[v][i];
			}
		}
	}
	for (int i = rr + d[u]; i < pt; i++) {
		f[u][i] = g[u][i] = 0;
	}
}

int main() {
	int n;
	cin >> n;
	for (int i = 1; i < n; ++i) {
		int a, b;
		cin >> a >> b;
		link(a, b);
		link(b, a); 
	}
	dfs1(1, 0);
	dfs(1, 0, 1);
	cout << ans;
} 

样例过不了