The 2019 ACM-ICPC China Shannxi Provincial Programming Contest （西安邀请赛重现） J. And And And

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const ll M = 2e5+10;
const ll inf = 1e18+10;
struct node{
    ll to,next,w;
}e[M];
const ll mod = 1000000007;
struct node1{
    ll num,id;
}Xor[M];
bool cmp(node1 x,node1 y){
    return x.num < y.num;
}
vector<ll>mp[M],v[M];
ll cnt,n,ans;
ll head[M],sz[M],d[M],md[M];
void add(ll u,ll v,ll w){
    e[++cnt].to = v;e[cnt].w = w;e[cnt].next = head[u];head[u] = cnt;
}

map<ll,ll>sum,sum1,num;

void get_dis(ll u,ll fa){
    Xor[++Xor[0].num].num = d[u];
    Xor[Xor[0].num].id = u;
    for(ll i = head[u];i;i=e[i].next){
        ll v = e[i].to;
        if(v != fa){
            d[v] = d[u]^e[i].w;
            get_dis(v,u);
        }
    }
    return ;
}

void get_siz(ll u,ll fa){
    sz[u] = 1;
    for(ll i = head[u];i;i=e[i].next){
        ll v = e[i].to;
        if(v != fa){
            get_siz(v,u);
            sz[u] += sz[v];
        }
    }
}
void gcd(ll a,ll b,ll &d,ll &x,ll &y)
{
    if(!b) {d=a;x=1;y=0;}
    else {gcd(b,a%b,d,y,x);y-=x*(a/b);}
}
ll finv(ll a,ll n)
{
  ll d,x,y;
  gcd(a,n,d,x,y);
  return d==1?(x+n)%n:-1;
}
void cal(ll u){
    d[u] = 0; Xor[0].num = 0;
    get_dis(u,0);
    sort(Xor+1,Xor+1+Xor[0].num,cmp);
    ll st = -1,idx = 0;
    for(ll i = 1;i <= Xor[0].num;i ++){
        if(Xor[i].num != st){
            st = Xor[i].num;
            mp[++idx].push_back(Xor[i].id);
            md[idx] = st;
        }
        else{
            mp[idx].push_back(Xor[i].id);
        }
    }
    ans = 0;
    for(ll i = 1;i <= idx;i ++){
        ll num1 = 0,num2 = 0;
        for(ll j = 0;j < mp[i].size();j ++){
            num1 += sz[mp[i][j]];
            num2 += sz[mp[i][j]]*sz[mp[i][j]]%mod;
            num1%=mod;     num2%=mod;
        }
        ans += ((num1*num1%mod+mod - num2)%mod)*finv(2,mod)%mod;
        ans %= mod;
    }
    for(ll i = 1;i <= idx;i ++) mp[i].clear();
}

void dfs(ll u,ll fa){
    for(ll i = head[u];i;i=e[i].next){
        ll v = e[i].to;
        if(v == fa) continue;
        sum1[d[u]] += (n - sz[v]+mod)%mod;
        if(num[d[v]] >= 1){
            ans = (ans + mod - (sz[v]*sum[d[v]]%mod))%mod;
            ans += sz[v]*sum1[d[v]]%mod;
            ans %= mod;
        }
        sum[d[v]] += sz[v];
        num[d[v]] += 1;
        sum[d[v]]%=mod;
        sum1[d[v]]%=mod;
        dfs(v,u);
        sum[d[v]] -= sz[v]-mod;
        sum1[d[u]] -= (n-sz[v])-mod;
        sum[d[v]]%=mod;
        sum1[d[v]]%=mod;
        num[d[v]] -= 1;
    }
}

int main()
{
    ll v,w;
    scanf("%lld",&n);
    for(ll i = 2;i <= n;i ++){
        scanf("%lld%lld",&v,&w);
        add(i,v,w); add(v,i,w);
    }
    get_siz(1,0);
    cal(1);
    sum[0] += sz[1];
    num[0] += 1;
    dfs(1,0);
    ans %= mod;
    num.clear(); sum.clear(); sum1.clear();
    printf("%lld\n",ans);
}