[CF1045C]Hyperspace Highways

题目大意：给一张$n$个点$m$条边的图，保证若有一个环，一定是完全子图，多次询问两个点之间的最短路径长度

题解：把完全子图缩成一个点，圆方树，方点权值设成$1$，圆点设成$0$即可。

卡点：数组开小

C++ Code：

#include <cstdio>
#include <algorithm>
#define maxn 100010
#define maxm 500010
inline int min(int a, int b) {return a < b ? a : b;}
inline int max(int a, int b) {return a > b ? a : b;}

namespace Tree {
	int head[maxn << 1], cnt;
	struct Edge {
		int to, nxt;
	} e[maxn << 2];
	inline void addE(int a, int b) {
		e[++cnt] = (Edge) {b, head[a]}; head[a] = cnt;
		e[++cnt] = (Edge) {a, head[b]}; head[b] = cnt;
	}

	#define M 19
	int nodenum, n;
	int dep[maxn << 1], sum[maxn << 1], fa[M][maxn << 1];
	void dfs(int u) {
		for (int i = 1; i < M; i++) fa[i][u] = fa[i - 1][fa[i - 1][u]];
		for (int i = head[u]; i; i = e[i].nxt) {
			int v = e[i].to;
			if (v != fa[0][u]) {
				fa[0][v] = u; dep[v] = dep[u] + 1;
				sum[v] = sum[u] + (v > n);
				dfs(v);
			}
		}
	}
	inline int LCA(int x, int y) {
		if (x == y) return x;
		if (dep[x] < dep[y]) std::swap(x, y);
		for (int i = dep[x] - dep[y]; i; i &= i - 1) x = fa[__builtin_ctz(i)][x];
		if (x == y) return x;
		for (int i = M - 1; ~i; i--) if (fa[i][x] != fa[i][y]) x = fa[i][x], y = fa[i][y];
		return fa[0][x];
	}
	inline int ask(int x, int y) {
		int lca = LCA(x, y);
		return sum[x] + sum[y] - (sum[lca] << 1) + (lca > n);
	}
	void init(int __n) {
		n = __n;
		sum[1] = 0;
		dfs(1);
	}
	#undef M
}

namespace Graph {
	int head[maxn], cnt;
	struct Edge {
		int to, nxt;
	} e[maxm << 1];
	inline void addE(int a, int b) {
		e[++cnt] = (Edge) {b, head[a]}; head[a] = cnt;
		e[++cnt] = (Edge) {a, head[b]}; head[b] = cnt;
	}

	using Tree::nodenum;
	int DFN[maxn], low[maxn], idx;
	int S[maxn], top;
	void tarjan(int u) {
		DFN[u] = low[u] = ++idx;
		S[++top] = u;
		for (int i = head[u]; i; i = e[i].nxt) {
			int v = e[i].to;
			if (!DFN[v]) {
				tarjan(v);
				low[u] = min(low[u], low[v]);
				if (low[v] >= DFN[u]) {
					nodenum++;
					Tree::addE(nodenum, u);
					do {
						v = S[top--];
						Tree::addE(nodenum, v);
					} while (v != e[i].to);
				}
			} else low[u] = min(low[u], DFN[v]);
		}
	}
	void init(int n) {
		tarjan(1);
		Tree::init(n);
	}
}

int n, m, Q;
int main() {
	scanf("%d%d%d", &n, &m, &Q); Tree::nodenum = Tree::n = n;
	for (int i = 0, a, b; i < m; i++) {
		scanf("%d%d", &a, &b);
		Graph::addE(a, b);
	}
	Graph::init(n);
	while (Q --> 0) {
		int u, v;
		scanf("%d%d", &u, &v);
		printf("%d\n", Tree::ask(u, v));
	}
	return 0;
}