POJ 1655 Balancing Act（求树的重心）

Description

Consider a tree T with N (1 <= N <= 20,000) nodes numbered 1...N. Deleting any node from the tree yields a forest: a collection of one or more trees. Define the balance of a node to be the size of the largest tree in the forest T created by deleting that node from T.
For example, consider the tree:

#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;

const int MAXN = 20010;
const int MAXE = 40010;
const int INF = 0x7fff7fff;

int head[MAXN], size[MAXN], maxSize[MAXN], f[MAXN];
int to[MAXE], next[MAXE];
int n, ecnt;

void init() {
memset(head, -1, sizeof(head));
ecnt = 0;
}

void add_edge(int u, int v) {
to[ecnt] = v; next[ecnt] = head[u]; head[u] = ecnt++;
to[ecnt] = u; next[ecnt] = head[v]; head[v] = ecnt++;
}

void dfs(int u) {
maxSize[u] = 0;
size[u] = 1;
for(int p = head[u]; ~p; p = next[p]) {
int &v = to[p];
if(v == f[u]) continue;
f[v] = u;
dfs(v);
size[u] += size[v];
maxSize[u] = max(maxSize[u], size[v]);
}
}

int main() {
int T;
scanf("%d", &T);
while(T--) {
init();
scanf("%d", &n);
int u, v;
for(int i = 1; i < n; ++i) {
scanf("%d%d", &u, &v);
add_edge(u, v);
}
dfs(1);
int pos, maxd = INF;
for(int i = 1; i <= n; ++i) {
if(max(maxSize[i], n - size[i]) < maxd) {
pos = i;
maxd = max(maxSize[i], n - size[i]);
}
}
printf("%d %d\n", pos, maxd);
}
}

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