[CF1000E]We Need More Bosses

题目大意：给一张无向图，要求找一对$s$和$t$，使得其路径上的割边是最多的，输出其数量。

题解：把边双缩点以后求树的直径。

卡点：无

C++ Code：

#include <cstdio>
#include <cstring>
#define maxn 300010
#define maxm 300010

#define ONLINE_JUDGE
#define read() R::READ()
#include <cctype>
namespace R {
	int x;
	#ifdef ONLINE_JUDGE
	char *ch, op[1 << 26];
	inline void init() {
		fread(ch = op, 1, 1 << 26, stdin);
	}
	inline int READ() {
		while (isspace(*ch)) ch++;
		for (x = *ch & 15, ch++; isdigit(*ch); ch++) x = x * 10 + (*ch & 15);
		return x;
	}
	#else
	char 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;
	}
	#endif
}

int n, m;
inline int min(int a, int b) {return a < b ? a : b;}

namespace Tree {
	int head[maxn], cnt;
	struct Edge {
		int to, nxt;
	} e[maxm << 1];
	void add(int a, int b) {
		e[++cnt] = (Edge) {b, head[a]}; head[a] = cnt;
		e[++cnt] = (Edge) {a, head[b]}; head[b] = cnt;
	}
	
	int q[maxn], h = 1, t = 0;
	int d[maxn];
	int bfs(int rt) {
		memset(d, 0, sizeof d);
		d[q[h = t = 0] = rt] = 1;
		while (h <= t) {
			int u = q[h++];
			for (int i = head[u]; i; i = e[i].nxt) {
				int v = e[i].to;
				if (!d[v]) {
					d[v] = d[u] + 1;
					q[++t] = v;
				}
			}
		}
		return q[t];
	}
	int work() {
		int x = bfs(1), y = bfs(x);
		return d[y] - 1;
	}
}
namespace Graph {
	int head[maxn], cnt;
	struct Edge {
		int from, to, nxt;
	} e[maxm << 1];
	void add(int a, int b) {
		e[++cnt] = (Edge) {a, b, head[a]}; head[a] = cnt;
		e[++cnt] = (Edge) {b, a, head[b]}; head[b] = cnt;
	}
	
	int res[maxn], CNT;
	int DFN[maxn], low[maxn], idx, fa[maxn];
	int S[maxn], top;
	void Tarjan(int u) {
		DFN[u] = low[u] = ++idx;
		S[++top] = u;
		int v;
		for (int i = head[u]; i; i = e[i].nxt) {
			v = e[i].to;
			if (v == fa[u]) continue;
			if (!DFN[v]) {
				fa[v] = u;
				Tarjan(v);
				low[u] = min(low[u], low[v]);
				if (low[v] > DFN[u]) {
					CNT++;
					do res[v = S[top--]] = CNT; while (v != e[i].to);
				}
			} else low[u] = min(low[u], DFN[v]);
		}
	}
	inline void tarjan(int n) {
		for (int i = 1; i <= n; i++) if (!DFN[i]) Tarjan(i);
	}
	inline void init() {
		for (int i = 1; i <= cnt; i += 2) {
			if (res[e[i].from] != res[e[i].to]) Tree::add(res[e[i].from], res[e[i].to]);
		}
	}
}

int main() {
	#ifdef ONLINE_JUDGE
	R::init();
	#endif
	n = read(), m = read();
	for (int i = 1; i <= m; i++) Graph::add(read(), read());
	Graph::tarjan(n);
	Graph::init();
	printf("%d
", Tree::work());
	return 0;
}