【NC204267】牛牛染顏色

題目鏈接：https://ac.nowcoder.com/acm/problem/204267
樹形dp
毒瘤題，卡vector和cin

#include<bits/stdc++.h>
#define rep(i,n) for(int i=1;i<=n;i++)
#define drep(i,n) for(int i=n;i;i--)
using namespace std;
 
const int N=1e6+5;
const int mod=1e9+7;
int n,m,k,x,y,z;
int head[N],num,dp[N];
 
struct EDGE {
    int to,next;
    EDGE(){}
} edge[N*2];
 
void add(int x,int y) {
    edge[++num].to=y;
    edge[num].next=head[x];
    head[x]=num;
}
 
int DP(int u,int f=0) {
    if(dp[u]) return dp[u];
    long long ans_a=1,ans_b=1;
    for(int i=head[u];i^-1;i=edge[i].next) {
        int v=edge[i].to;
        if(v==f) continue;
        ans_a*=DP(v,u);
        ans_b+=DP(v,u)-1;
        ans_a%=mod,ans_b%=mod;
    }
    return dp[u]=(ans_a+ans_b)%mod;
}
 
void solve() {
    scanf("%d",&n);
    rep(i,n) head[i]=-1;
    rep(i,n-1) {
        scanf("%d%d",&x,&y);
        add(x,y),add(y,x);
    }
    printf("%d\n",DP(1));
}
 
int main() {
    solve();
}