记录编号	310264	评测结果	AAAAAAAAAA
题目名称	[SYOI 2015] Asm.Def的基本算法	最终得分	100
用户昵称	Fmuckss	是否通过	通过
代码语言	C++	运行时间	0.143 s
提交时间	2016-09-22 07:36:37	内存使用	3.36 MiB

#include <iostream>
#include <cstdio>
using namespace std;
typedef long long LL;
const int maxn = 1e5 + 10;
const int lcy = 1e9 + 7;


struct Edge {
	int to, ne;
	Edge() {}
	Edge(int to_, int ne_) { to = to_, ne = ne_; }
} e[maxn << 1];
int head[maxn], ecnt;
void add_edge(int fr, int to) {
	e[++ecnt] = Edge(to, head[fr]), head[fr] = ecnt;
}


#define is_num(x) (x <= '9' and x >= '0')
inline int get_num() {
	char tmp = getchar();
	int res = 0;
	
	while (not is_num(tmp)) tmp = getchar();
	while (    is_num(tmp)) {
		res = (res << 3) + (res << 1) + tmp - '0';
		tmp = getchar();
	}
	
	return res;
} 


int sum[maxn];
int sz[maxn];
int v[maxn];
int n;
LL ans = 0;


void dfs(int now, int fr) {
	sz[now] = 1;
	sum[now] = v[now];
	
	for (int i = head[now]; i; i = e[i].ne) {
		int to = e[i].to;
		if (to == fr) continue;
		
		dfs(to, now);
		sum[now] = (sum[now] + sum[to]) % lcy;
		sz[now] += sz[to];
	}
	
	LL tot_s = sum[now];
	LL tmp = 0;
	for (int i = head[now]; i; i = e[i].ne) {
		int to = e[i].to;
		if (to == fr) continue;
		tmp = (tmp + (1ll * sum[to] * ((tot_s - sum[to] + lcy) % lcy) % lcy)) % lcy;
		tmp = (tmp + (1ll * sum[to] * v[now] % lcy)) % lcy;
	}
	
	tmp = (tmp + (1ll * v[now] * v[now] % lcy)) % lcy;
	tmp = (1ll * tmp * v[now]) % lcy;
	ans = (ans + tmp) % lcy;
}


inline void read() {
	n = get_num();
	v[1] = get_num();
	int fa;
	for (int i = 2; i <= n; i++) {
		fa = get_num();
		v[i] = get_num();
		add_edge(fa, i);
	}
}


inline void solve() {
	dfs(1, 0);
	printf("%lld\n", ans);
}


int main() {
	freopen("asm_algo.in", "r", stdin);
	freopen("asm_algo.out", "w", stdout);
	read();
	solve();
	return 0;
}