CodeForces 487E Tourists（圆方树+线段树+树链剖分）

题意

​ (n) 个点 (m) 条边的无向连通图，每个点有点权，(q) 个要求，每次更新一个点的点权或查询两点间路径权值最小的点最小的路径。

思路

​ 算是圆方树的板子吧？圆方树处理的主要就是两点之间路径的问题。

​ 我们先对原图建一棵圆方树，然后每个圆点的信息传递给父亲，一定是方点，用堆维护信息。最后只要树剖线段树查路径最小值即可，注意特判 ( m{lca}) 即可。

代码

#include<bits/stdc++.h>
#define FOR(i, x, y) for(int i = (x), i##END = (y); i <= i##END; ++i)
#define DOR(i, x, y) for(int i = (x), i##END = (y); i >= i##END; --i)
template<typename T, typename _T> inline bool chk_min(T &x, const _T &y) {return y < x ? x = y, 1 : 0;}
template<typename T, typename _T> inline bool chk_max(T &x, const _T &y) {return x < y ? x = y, 1 : 0;}
typedef long long ll;
const int N = 100005;
const int M = 100005;

template<const int N, const int M, typename T> struct Linked_List
{
	int head[N], nxt[M], tot; T to[M];
	Linked_List() {clear();}
	T &operator [](const int x) {return to[x];}
	void clear() {memset(head, -1, sizeof(head)), tot = 0;}
	void add(int u, T v) {to[tot] = v, nxt[tot] = head[u], head[u] = tot++;}
	#define EOR(i, G, u) for(int i = G.head[u]; ~i; i = G.nxt[i])
};


struct Segment_Tree
{
	int mi[N << 3];
	void update(int k, int x, int val, int l, int r)
	{
		if(l == r) {mi[k] = val; return;}
		int mid = (l + r) >> 1;
		if(x <= mid) update(k << 1, x, val, l, mid);
		else update(k << 1 | 1, x, val, mid + 1, r);
		mi[k] = std::min(mi[k << 1], mi[k << 1 | 1]);
	}
	int query(int k, int L, int R, int l, int r)
	{
		if(L <= l && r <= R) return mi[k];
		int mid = (l + r) >> 1;
		if(R <= mid) return query(k << 1, L, R, l, mid);
		else if(L > mid) return query(k << 1 | 1, L, R, mid + 1, r);
		else return std::min(query(k << 1, L, R, l, mid), query(k << 1 | 1, L, R, mid + 1, r));
	}
};

Linked_List<N, M << 1, int> G;
Linked_List<N << 1, N << 2, int> T;
Segment_Tree ST;

int dfn[N], low[N], stk[N], idxer, bcc, tp;

int fa[N << 1], dep[N << 1], sz[N << 1], son[N << 1], top[N << 1];
int lfn[N << 1], rfn[N << 1], ori[N << 1], dfn_idx;
int pw[N << 1];
std::multiset<int> st[N << 1];

int n, m, q;

void tarjan(int u, int fa_e)
{
	dfn[u] = low[u] = ++idxer;
	stk[++tp] = u;

	EOR(i, G, u)
	{
		if(i == (fa_e ^ 1)) continue;
		int v = G[i];
		if(!dfn[v])
		{
			tarjan(v, i), chk_min(low[u], low[v]);
			if(low[v] >= dfn[u])
			{
				bcc++;
				do
				{
					T.add(n + bcc, stk[tp]);
					T.add(stk[tp], n + bcc);
				}
				while(stk[tp--] != v);
				T.add(n + bcc, u), T.add(u, n + bcc);
			}
		}
		else if(dfn[v] < dfn[u])
			chk_min(low[u], dfn[v]);
	}
}

void dfs(int u, int f, int d)
{
	fa[u] = f, dep[u] = d, sz[u] = 1, son[u] = 0;
	EOR(i, T, u)
	{
		int v = T[i];
		if(v == f) continue;
		dfs(v, u, d + 1);
		sz[u] += sz[v];
		if(sz[v] > sz[son[u]]) son[u] = v;
	}
}

void hld(int u, int tp)
{
	ori[lfn[u] = ++dfn_idx] = u;
	top[u] = tp;
	if(son[u]) hld(son[u], tp);
	EOR(i, T, u)
	{
		int v = T[i];
		if(v == fa[u] || v == son[u]) continue;
		hld(v, v);
	}
	rfn[u] = dfn_idx;
}

int get_lca(int u, int v)
{
	while(top[u] != top[v])
	{
		if(dep[top[u]] < dep[top[v]]) std::swap(u, v);
		u = fa[top[u]];
	}
	return dep[u] < dep[v] ? u : v;
}

int query(int u, int v)
{
	int res = 2e9;
	while(top[u] != top[v])
	{
		if(dep[top[u]] < dep[top[v]]) std::swap(u, v);
		chk_min(res, ST.query(1, lfn[top[u]], lfn[u], 1, n + bcc));
		u = fa[top[u]];
	}
	if(dep[u] > dep[v]) std::swap(u, v);
	chk_min(res, ST.query(1, lfn[u], lfn[v], 1, n + bcc));
	if(u > n) chk_min(res, pw[fa[u]]);
	return res;
}

int main()
{
	scanf("%d%d%d", &n, &m, &q);
	FOR(i, 1, n) scanf("%d", &pw[i]);
	FOR(i, 1, m)
	{
		int u, v;
		scanf("%d%d", &u, &v);
		G.add(u, v), G.add(v, u);
	}

	tarjan(1, -1);
	dfs(1, 0, 1), hld(1, 1);

	FOR(u, n + 1, n + bcc)
	{
		EOR(i, T, u)
		{
			int v = T[i];
			if(v == fa[u]) continue;
			st[u].insert(pw[v]);
		}
		pw[u] = (*st[u].begin());
	}
	
	FOR(i, 1, n + bcc) ST.update(1, lfn[i], pw[i], 1, n + bcc);

	while(q--)
	{
		char str[3]; int a, b;
		scanf("%s%d%d", str, &a, &b);
		if(str[0] == 'C')
		{
			if(fa[a])
			{
				st[fa[a]].erase(st[fa[a]].find(pw[a]));
				st[fa[a]].insert(b);
				pw[fa[a]] = (*st[fa[a]].begin());
				ST.update(1, lfn[fa[a]], pw[fa[a]], 1, n + bcc);
			}
			pw[a] = b;
			ST.update(1, lfn[a], b, 1, n + bcc);
			
		}
		else if(str[0] == 'A')
			printf("%d
", query(a, b));
	}

	return 0;
}