记录编号	243748	评测结果	AAAAAAAAAA
题目名称	[HAOI 2009]毛毛虫	最终得分	100
用户昵称	Fmuckss	是否通过	通过
代码语言	C++	运行时间	0.573 s
提交时间	2016-03-30 17:24:35	内存使用	15.48 MiB

#include<iostream>
#include<cstdio>
using namespace std;
const int maxn = 3e5+50;
int f[maxn];
int ans;
int n, m; 
int d[maxn];

template <class T> class stack {
private:
	T a[maxn];
	int top_;
public:
	inline stack() { top_ = 0; } 
	inline void push(T tar) { a[++top_] = tar; }
	inline void pop() { top_--; }
	inline T top() { return a[top_]; }
	inline bool empty() { return !top_; }
};

struct edge {
	int to, ne;
	inline edge() {}
	inline edge(int to_, int ne_) { to = to_, ne = ne_; }
}e[maxn*2];
int head[maxn], tot;


inline int get_num() {
	int ans = 0;
	char tmp = getchar();
	while(tmp < '0' || tmp > '9') tmp = getchar();
	while(tmp <= '9' && tmp >= '0') {
		ans = 10*ans + tmp - '0';
		tmp = getchar();
	}
	return ans;
} 


inline void add_edge(int fr, int to) {
	e[++tot] = edge(to, head[fr]); head[fr] = tot; d[fr]++;
	e[++tot] = edge(fr, head[to]); head[to] = tot; d[to]++;
}


inline void read() {
	n = get_num(); m = get_num();
	for(int i = 1; i <= m; i++) add_edge(get_num(), get_num()); 
} 

struct dfs {
	int fa, now, i, mx, se;
	bool be;
	dfs() {}
	dfs(int fa_, int now_, int i_, int mx_, int se_, bool be_) {
		fa = fa_, now = now_, i = i_, mx = mx_, se = se_, be = be_;
	}
};

stack <dfs> s;
bool vis[maxn];
inline void dp() {
	s.push(dfs(0, 1, head[1], 0, 0, true));
	int i, mx, se, fa;
	dfs now;
	while(!s.empty()) {
		now = s.top(); s.pop();
		fa = now. fa, mx = now.mx, se = now.se, i = now.i;
		vis[now.now] = true;
		if(now.be) {
			s.push(dfs(fa, now.now, i, mx, se, false)); 
			int to = e[i].to;
			if(to == fa) continue;
			s.push(dfs(now.now, to, head[to], 0, 0, true));
		} else {
			int to = e[i].to;
			if(to != fa) {
				if(f[to] > mx) se = mx, mx = f[to];
				else if(f[to] > se) se = f[to];
			}
			if(!e[i].ne) {
				f[now.now] = mx+d[now.now]-1;
				ans = max(ans, se + f[now.now] + 1);
				continue;
			} else {
				s.push(dfs(fa, now.now, e[i].ne, mx, se, true));
			}
		}
	}
//	for(int i = 1; i <= n; i++) {
//		printf("%d ", f[i]);
//	}
}


int main() {
	freopen("worma.in", "r", stdin);
	freopen("worma.out", "w", stdout);
	read();
	dp();
	printf("%d", ans+1);//加上的1是根节点本身(根节点不会到达他的父节点) 
	return 0;
}