【算法练习】CodeVs3639 树的重心

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题意

输出删除树的中心后，子树最大的size数目，如果有多个树的重心，升序输出他们的值。

题解

dfs一遍。
记录当前节点的size和子树最大的size，另一半就是nodenum - size[u]，取个最大值就是子树最大的size数目。一边统计一遍更新答案。

代码

#include<bits/stdc++.h>
using namespace std;
typedef double db;
typedef long long ll;
typedef unsigned long long ull;
const int nmax = 1e6+7;
const int INF = 0x3f3f3f3f;
const ll LINF = 0x3f3f3f3f3f3f3f3f;
const ull p = 67;
const ull MOD = 1610612741;
struct edge {
    int to, nxt;
}e[nmax<<1];
int head[nmax], tot;
int sz[nmax];
int nowmin = INF;
vector<int> vertex;
void add_edge(int u, int v) {
    e[tot].to = v;
    e[tot].nxt = head[u];
    head[u] = tot++;
}
int n;
void dfs(int u, int f) {
    sz[u] = 1;
    int mxchild = 0; 
    for(int i = head[u]; i != -1; i = e[i].nxt) {
        int v = e[i].to;
        if(v != f) {
            dfs(v, u);
            mxchild = max(mxchild, sz[v]);
            sz[u] += sz[v];
        }
    }
    int tmp = max(mxchild, n - sz[u]);
    if(tmp < nowmin) {
        vertex.clear();
        nowmin = tmp;
        vertex.push_back(u);
    } else if (tmp == nowmin) {
        vertex.push_back(u);
    }
}
int main(){
    memset(head , -1, sizeof head);
    scanf("%d", &n);
    for(int i = 1; i <= n - 1; ++i) {
        int u, v;
        scanf("%d %d", &u, &v);
        add_edge(u, v);
        add_edge(v, u);
    }
    dfs(1, -1);
    printf("%d %d\n", nowmin, vertex.size());
    sort(vertex.begin(), vertex.end());
    for(int i = 0; i < vertex.size() ; ++i) {
        printf("%d ",vertex[i]);
    }
    printf("\n");
    return 0;
}