[洛谷P3174][HAOI2009]毛毛虫

题目大意：给一棵树，求其中最大的“毛毛虫”，毛毛虫的定义是一条链上分出几条边

题解：把每个点的权值定义为它的度数减一，跑带权直径即可，最后答案加二

卡点：无

C++ Code：

#include <cstdio>
#include <cctype>
namespace __IO {
	namespace R {
		int x, ch;
		inline int read() {
			ch = getchar();
			while (isspace(ch)) ch = getchar();
			for (x = ch & 15, ch = getchar(); isdigit(ch); ch = getchar()) x = x * 10 + (ch & 15);
			return x;
		}
	}
}
using __IO::R::read;

inline int max(int a, int b) {return a > b ? a : b;}

#define maxn 300010
int head[maxn], cnt;
struct Edge {
	int to, nxt;
} e[maxn << 1];
inline void add(int a, int b) {
	e[++cnt] = (Edge) {b, head[a]}; head[a] = cnt;
}

int n, m;
int w[maxn];
int max1[maxn], max2[maxn], ans = -20040826;
int dfs(int u, int fa = 0) {
	for (int i = head[u]; i; i = e[i].nxt) {
		int v = e[i].to;
		if (v != fa) {
			int tmp = dfs(v, u);
			if (tmp > max1[u]) {
				max2[u] = max1[u];
				max1[u] = tmp;
			} else if (tmp > max2[u]) max2[u] = tmp;
		}
	}
	max1[u] += w[u];
	ans = max(ans, max1[u] + max2[u]);
	return max1[u];
}
int main() {
	n = read(), m = read();
	for (int i = 1, a, b; i < n; i++) {
		a = read(), b = read();
		add(a, b);
		w[a]++, w[b]++;
	}
	for (int i = 1; i <= n; i++) w[i]--;
	dfs(1);
	printf("%d\n", ans + 2);
	return 0;
}