百练3383:Cell Phone Network

# include <stdio.h>
# include <string.h>
# include <iostream>
# include <algorithm>
// # include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
const int M = 5e5 + 10;
const int mod = 1e9+7;

# define RG register
# define ST static
/*
首先选择一点为树根,再按照深度优先遍历得到遍历序列,按照所得序列
的反向序列的顺序进行贪心,对于一个即不属于支配集也不与支配集中的
点相连的点来说,如果他的父节点不属于支配集,将其父节点加入到支配集
*/

int n;
int head[M], nxt[M], to[M], tot=0;
bool s[M], is[M]; // have been covered yet(in set or connected with ... in set),  in set 

inline void add(int u, int v) {
    ++tot; nxt[tot] = head[u];
    head[u] = tot; to[tot] = v;
}
inline void adde(int u, int v) {
    add(u, v), add(v, u);
}

int dfn[M], DFN=0, fa[M]; 
inline void dfs(int x, int fat) {
    ++DFN; fa[x] = fat; 
    dfn[DFN] = x;
    for (int i=head[x]; i; i=nxt[i]) 
        if(to[i] != fat) dfs(to[i], x); 
}

int main() {
    while(cin >> n) { 
        memset(head, 0, sizeof head); tot = 0;
        DFN = 0; memset(s, 0, sizeof s); memset(is, 0, sizeof is);
        memset(fa, 0, sizeof fa); 
        for (int i=1, u, v; i<n; ++i) {
            scanf("%d%d", &u, &v);
            adde(u, v);  
        }
        dfs(1, 1);
        for (int i=n; i>=1; --i) {
            int x = dfn[i]; 
            if(!s[x]) {
                if(!is[fa[x]]) is[fa[x]] = 1; 
                s[x] = 1; s[fa[x]] = 1; s[fa[fa[x]]] = 1;
            }
        }
        int ans = 0;
        for (int i=1; i<=n; ++i) ans += is[i]; 
        if(n == 1) puts("1");
        else printf("%d\n", ans); 
    }
    return 0;
}