题目链接：https://www.luogu.org/problemnew/show/P3979
除了换根操作都是裸的树剖
所以换根时考虑：
1.我查询的根等于换的根：无影响
2.我查询的根是换的根的子树：无影响
3.我查询的根是换的根的祖先：查询 除换的根及其往上直到为要查询的根的第一层儿子的祖先(设为S)的子树 以外的所有节点 (此时满足seg[S] <= seg[root] <= seg[S]+size[S]-1)
求LCA 查询1到seg[S]-1 和 seg[S]+size[S]到n 正好避开子树seg[S] 到 seg[S]+size[S]-1

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#define ll long long
#define lson left, mid, rt<<1
#define rson mid + 1, right, rt<<1|1
using namespace std;
const ll maxn = 200000 + 10;
ll tree[maxn<<2], lazy[maxn<<2];
ll seg[maxn], fa[maxn], size[maxn], rev[maxn], son[maxn], deep[maxn], top[maxn], num;
ll node[maxn], res, n, m, root, r;
struct edge{ll from, to, next;
}e[maxn<<2];
ll head[maxn], cnt;
void add(ll u, ll v)
{e[++cnt].from = u;e[cnt].next = head[u];e[cnt].to = v;head[u] = cnt;
}
//--------------------------segment_tree
void PushUP(ll rt)
{tree[rt] = min(tree[rt<<1], tree[rt<<1|1]);
}
void build(ll left, ll right, ll rt)
{if(left == right){tree[rt] = rev[left];return;}ll mid = (left + right)>>1;build(lson);build(rson);PushUP(rt);
}
void PushDOWN(ll rt)
{lazy[rt<<1] = lazy[rt];lazy[rt<<1|1] = lazy[rt];tree[rt<<1] = lazy[rt];tree[rt<<1|1] = lazy[rt];lazy[rt] = 0;
}
void update(ll l, ll r, ll add, ll left, ll right, ll rt)
{if(l <= left && r >= right){tree[rt] = add;lazy[rt] = add;return;}ll mid = (left + right)>>1;if(lazy[rt]) PushDOWN(rt);if(l <= mid) update(l, r, add, lson);if(r > mid) update(l, r, add, rson);PushUP(rt);
}
ll query(ll l, ll r, ll left, ll right, ll rt)
{ll res = 0x7fffffff;if(l <= left && r >= right){return tree[rt];}ll mid = (left + right)>>1;if(lazy[rt]) PushDOWN(rt);if(l <= mid) res = min(res, query(l, r, lson));if(r > mid) res = min(res, query(l, r, rson));return res;
}
//------------------------
void dfs1(ll u, ll f, ll d)
{ll maxson = -1;size[u] = 1;deep[u] = d;fa[u] = f;for(ll i = head[u]; i != -1; i = e[i].next){ll v = e[i].to;if(f != v){dfs1(v, u, d+1);size[u] += size[v];if(size[v] > maxson) son[u] = v, maxson = size[v];}}
}
void dfs2(ll u, ll t)
{seg[u] = ++num;rev[num] = node[u];top[u] = t;if(!son[u]) return;dfs2(son[u], t);for(ll i = head[u]; i != -1; i = e[i].next){ll v = e[i].to;if(v == son[u] || v == fa[u]) continue;dfs2(v,v);}
}
/*ll qRange(ll x, ll y)
{ll ans = 0;while(top[x] != top[y]){if(deep[top[x]] < deep[top[y]]) swap(x, y);res = 0;res = query(seg[top[x]], seg[x], 1, n, 1);ans = (ans + res);x = fa[top[x]];}if(deep[x] > deep[y]) swap(x, y);res = 0;res = query(seg[x], seg[y], 1, n, 1);ans = (ans + res);return ans;
}*/
void updRange(ll x, ll y, ll k)
{while(top[x] != top[y]){if(deep[top[x]] < deep[top[y]]) swap(x, y);update(seg[top[x]], seg[x], k, 1, n, 1);x = fa[top[x]];}if(deep[x] > deep[y]) swap(x, y);update(seg[x], seg[y], k, 1, n, 1);
}
ll LCA(int x, int y)
{while(top[x] != top[y]){if(deep[top[x]] < deep[top[y]]) swap(x, y);x = fa[top[x]];}return deep[x] < deep[y] ? x : y;
}
ll qSon(ll x)
{if(x == r)return query(1, n, 1, n, 1);int p = LCA(r, x);if(p != x)return query(seg[x], seg[x]+size[x]-1, 1, n, 1);int S;for(int i = head[x]; i != -1; i = e[i].next){int v = e[i].to;if(seg[v] <= seg[r] && seg[r] <= seg[v] + size[v] -1 && v != fa[x]){S = v; break;}}return min(query(1, seg[S]-1, 1, n, 1), query(seg[S]+size[S], n, 1, n, 1));
}
int main()
{memset(head, -1, sizeof(head));scanf("%lld%lld",&n,&m);for(ll i = 1; i < n; i++){ll u, v;scanf("%lld%lld",&u,&v);add(u, v), add(v, u);}for(ll i = 1; i <= n; i++)scanf("%lld",&node[i]);scanf("%lld",&root);dfs1(root, 0, 1);dfs2(root, root);build(1,n,1);r = root;for(ll i = 1; i <= m; i++){ll opt, x, y, z;scanf("%lld",&opt);if(opt == 1){scanf("%lld",&x);r = x;}if(opt == 2){scanf("%lld%lld%lld",&x,&y,&z);updRange(x, y, z);}if(opt == 3){scanf("%lld",&x);printf("%lld\n",qSon(x));}}return 0;
}