洛谷P2996 [USACO10NOV]拜访奶牛Visiting Cows

//设f[i][j]表示选到第i号节点的最大权值 
#include <cstdio>
#include <iostream>
using namespace std;
const int N = 500010;
int f[N][2], n, head[N << 1], cnt;
struct node {
    int nxt, to;
}e[N];
int read() {
    int s = 0, w = 1;
    char ch = getchar();
    while(!isdigit(ch)) {if(ch == '-') w = -1; ch = getchar();}
    while(isdigit(ch)) {s = s * 10 + ch - '0'; ch = getchar();}
    return s * w;
}
void add(int x, int y) {
    e[++cnt].nxt = head[x];
    e[cnt].to = y;
    head[x] = cnt;
}
void dfs(int x, int fa) {
    f[x][1] = 1;
    for(int i = head[x]; i; i = e[i].nxt) {
        int v = e[i].to;
        if(v != fa) {
            dfs(v, x);
            f[x][0] += max(f[v][0], f[v][1]);
            f[x][1] += f[v][0];    
        } 
    }
}
int main() {
    n = read();
    for(int i = 1, x, y; i < n; i++) {
        x = read(), y = read();
        add(x, y), add(y, x);
    }
    dfs(1, 0);
    cout << max(f[1][1], f[1][0]) << endl;
    return 0;
}