P1352 没有上司的舞会

树形DP第一道

看这里:https://www.luogu.org/problemnew/show/P1352

#include <bits/stdc++.h>
#define read read()
using namespace std;

const int N = 10000;

int n,size;
int head[N],a[N];
int f[N][2];

int read
{
    int x = 0,f = 1; char ch = getchar();
    while( ch < 48 || ch > 57 ) {if(ch == '-') f = -1; ch = getchar();}
    while(ch >= 48 && ch <= 57) {x = 10 * x + ch - 48; ch = getchar();}
    return x * f;
}

struct edge{
    int v,nxt;
}e[N<<1];

void add(int u,int v)
{
    e[++size].v = v;
    e[size].nxt = head[u];
    head[u] = size;
}

void dfs(int u,int fa)
{
    f[u][0] = 0; f[u][1] =  a[u];
    for(int i = head[u]; i; i = e[i].nxt)
    {
        int v = e[i].v;
        if(v == fa) continue;
        dfs(v,u);
        f[u][0] += max(f[v][0] , f[v][1]);
        f[u][1] += f[v][0];
    }
}

int main()
{
//    freopen("boos.in","r",stdin);
    n = read;
    for(int i = 1; i <= n; i++) a[i] = read;
    int u,v;
    for(int i = 1; i < n; i++)
    {
        v = read; u = read;
        add(u,v);
        add(v,u);
    }
    u = read; v = read;
    dfs(1,0);
    printf("%d",max(f[1][0] , f[1][1]));
    return 0;
}