Zero Tree CodeForces - 275D

http://codeforces.com/problemset/problem/275/D

minn[i]代表以i为根的子树要变零树要减几次 maxx[i]是加几次 最后汇总至根节点输出maxx[1]-minn[1]即可

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll N=0x3f3f3f3f3f3f3f3f;
const int maxn=1e5+10;

struct node
{
    int v,next;
};

node edge[2*maxn];
ll val[maxn],minn[maxn],maxx[maxn];
int first[maxn];
int n,num;

void addedge(int u,int v)
{
    edge[num].v=v;
    edge[num].next=first[u];
    first[u]=num++;
}

void dfs(int cur,int fa)
{
    int i,v;
    for(i=first[cur];i!=-1;i=edge[i].next){
        v=edge[i].v;
        if(v!=fa){
            dfs(v,cur);
            minn[cur]=min(minn[cur],minn[v]);
            maxx[cur]=max(maxx[cur],maxx[v]);
        }
    }
    val[cur]-=maxx[cur]+minn[cur];
    if(val[cur]>0) maxx[cur]+=val[cur];
    else minn[cur]+=val[cur];
}

int main()
{
    int i,u,v;
    scanf("%d",&n);
    memset(first,-1,sizeof(first));
    num=0;
    for(i=1;i<=n-1;i++){
        scanf("%d%d",&u,&v);
        addedge(u,v);
        addedge(v,u);
    }
    for(i=1;i<=n;i++){
        scanf("%lld",&val[i]);
    }
    dfs(1,0);
    printf("%lld\n",maxx[1]-minn[1]);
    return 0;
}