【POJ1655】Balancing Act【树的重心】

题意：

求树的重心。

做点分治前先做的一个题。

#include <cstdio>
#include <algorithm>

using namespace std;

const int maxn = 20005, inf = 0x3f3f3f3f;

int n, head[maxn], cnt, size[maxn], id, ans;

struct _edge {
	int v, next;
} g[maxn << 1];

inline int iread() {
	int f = 1, x = 0; char ch = getchar();
	for(; ch < '0' || ch > '9'; ch = getchar()) f = ch == '-' ? -1 : 1;
	for(; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - '0';
	return f * x;
}

inline void add(int u, int v) {
	g[cnt] = (_edge) {v, head[u]};
	head[u] = cnt++;
}

void dfs(int x, int f) {
	size[x] = 1;
	int res = -1;
	for(int i = head[x]; ~i; i = g[i].next) if(g[i].v ^ f) {
		dfs(g[i].v, x);
		size[x] += size[g[i].v];
		res = max(res, size[g[i].v]);
	}
	res = max(res, n - size[x]);
	if(res < ans || (res == ans && id >= x)) {
		ans = res;
		id = x;
	}
}

int main() {
	int T = iread();
	while(T--) {
		n = iread();
		for(int i = 1; i <= n; i++) head[i] = -1; cnt = 0;

		for(int i = 1; i < n; i++) {
			int u = iread(), v = iread();
			add(u, v); add(v, u);
		}

		id = ans = inf;
		dfs(1, 0);

		printf("%d %d\n", id, ans);
	}
	return 0;
}