rt，我有一发代码是这样打的：

#include<cstdio>
#include<algorithm>
using namespace std;

#define rep(i,__l,__r) for(signed i=(__l),i##_end_=(__r);i<=i##_end_;++i)
#define fep(i,__l,__r) for(signed i=(__l),i##_end_=(__r);i>=i##_end_;--i)
#define erep(i,u) for(signed i=tail[u],v=e[i].to;i;i=e[i].nxt,v=e[i].to)
#define writc(a,b) fwrit(a),putchar(b)
#define mp(a,b) make_pair(a,b)
#define fi first
#define se second
typedef long long LL;
// typedef pair<int,int> pii;
typedef unsigned long long ull;
typedef unsigned uint;
#define Endl putchar('\n')
// #define int long long
//#define int unsigned
// #define int unsigned long long

#define cg (c=getchar())
template<class T>inline void read(T& x){
    char c;bool f=0;
    while(cg<'0'||'9'<c)f|=(c=='-');
    for(x=(c^48);'0'<=cg&&c<='9';x=(x<<1)+(x<<3)+(c^48));
    if(f)x=-x;
}
template<class T>inline T read(const T sample){
    T x=0;char c;bool f=0;
    while(cg<'0'||'9'<c)f|=(c=='-');
    for(x=(c^48);'0'<=cg&&c<='9';x=(x<<1)+(x<<3)+(c^48));
    return f?-x:x;
}
template<class T>void fwrit(const T x){//just short,int and long long
    if(x<0)return (void)(putchar('-'),fwrit(-x));
    if(x>9)fwrit(x/10);
    putchar(x%10^48);
}
template<class T>inline T Max(const T x,const T y){return x<y?y:x;}
template<class T>inline T Min(const T x,const T y){return x<y?x:y;}
template<class T>inline T fab(const T x){return x>0?x:-x;}
inline int gcd(const int a,const int b){return b?gcd(b,a%b):a;}
inline void getInv(int inv[],const int lim,const int MOD){
    inv[0]=inv[1]=1;for(int i=2;i<=lim;++i)inv[i]=1ll*inv[MOD%i]*(MOD-MOD/i)%MOD;
}
inline LL mulMod(const LL a,const LL b,const LL mod){//long long multiplie_mod
    return ((a*b-(LL)((long double)a/mod*b+1e-8)*mod)%mod+mod)%mod;
}

const int maxn=100000;
const uint inf=(1<<31)+5;

struct edge{int to,nxt;}e[maxn*2+5];
int tail[maxn+5],ecnt;
inline void add_edge(const int u,const int v){
    e[++ecnt]=edge{v,tail[u]};tail[u]=ecnt;
    e[++ecnt]=edge{u,tail[v]};tail[v]=ecnt;
}

int n,m,rt;

uint a[maxn+5];

inline void Init(){
    n=read(1),m=read(1);
    rep(i,1,n-1)add_edge(read(1),read(1));
    rep(i,1,n)a[i]=read(1u);
    rt=read(1);
}

int fa[maxn+5],d[maxn+5],sz[maxn+5],son[maxn+5];

void dfs1(const int u,const int pre){
    fa[u]=pre,d[u]=d[pre]+1,sz[u]=1;
    erep(i,u)if(v^pre){
        dfs1(v,u);
        sz[u]+=sz[v];
        if(sz[v]>sz[son[u]])son[u]=v;
    }
}

int top[maxn+5],dfn[maxn+5],times;
uint origin[maxn+5];

void dfs2(const int u,const int bel){
    top[u]=bel,dfn[u]=++times;
    origin[times]=a[u];
    if(!son[u])return;
    dfs2(son[u],bel);
    erep(i,u)if(v!=fa[u] && v!=son[u])
        dfs2(v,v);
}

struct segment_tre{
    uint minn[maxn<<2|2];
    uint tag[maxn<<2|2];
    #define lc (i<<1)
    #define rc (i<<1|1)
    #define mid (l+r>>1)
    #define _lq lc,l,mid
    #define _rq rc,mid+1,r
    inline void pushup(const int i){
        minn[i]=Min(minn[lc],minn[rc]);
    }
    inline void update(const int i,const uint x){
        minn[i]=x,tag[i]=x;
    }
    inline void pushdown(const int i){
        update(lc,tag[i]),update(rc,tag[i]);
        tag[i]=0;
    }
    void build(const int i=1,const int l=1,const int r=n){
        if(l==r)return minn[i]=origin[l],void();
        build(_lq),build(_rq);
        pushup(i);
    }
    void modify(const int L,const int R,const uint x,const int i=1,const int l=1,const int r=n){
        // printf("modify:>L == %d, R == %d\n",L,R);   
        if(L<=l && r<=R)return update(i,x);
        if(tag[i])pushdown(i);
        if(L<=mid)modify(L,R,x,_lq);
        if(mid<R)modify(L,R,x,_rq);
        pushup(i);
    }
    uint query(const int L,const int R,const int i=1,const int l=1,const int r=n){
        if(L<=l && r<=R)return minn[i];
        if(tag[i])pushdown(i);
        uint ret=inf;
        if(L<=mid)ret=query(L,R,_lq);
        if(mid<R)ret=Min(ret,query(L,R,_rq));
        return ret;
    }
}tre;

inline void modify(int u,int v,const uint x){
    while(top[u]!=top[v]){
        if(d[top[v]]<d[top[v]])swap(u,v);
        tre.modify(dfn[top[u]],dfn[u],v);
        u=fa[top[u]];
    }if(dfn[u]>dfn[v])swap(u,v);
    tre.modify(dfn[u],dfn[v],x);
}

signed main(){
    Init();
    dfs1(1,0);
    dfs2(1,1);
    // rep(i,1,n)printf("dfn[%d] == %d, son[%d] == %d, sz[%d] == %d\n",i,dfn[i],i,son[i],i,sz[i]);
    tre.build();
    int opt,id,p1,p2;uint v;
    while(m--){
        opt=read(1);
        if(opt==1)rt=read(1);
        else if(opt==2){
            p1=read(1),p2=read(1),v=read(1u);
            modify(p1,p2,v);
        }else{
            id=read(1);
            if(id==rt)writc(tre.minn[1],'\n');
            else if(dfn[id]<=dfn[rt] && dfn[rt]<=dfn[id]+sz[id]-1){
                //如果根在询问点的子树内
                uint ans;
                erep(i,id)if(i!=fa[id] && dfn[v]<=dfn[rt] && dfn[rt]<=dfn[v]+sz[v]-1){
                    //查找是在哪颗子树内
                    ans=Min(tre.query(1,dfn[v]-1),tre.query(dfn[v]+sz[v],n));
                    break;
                }writc(ans,'\n');
            }else writc(tre.query(dfn[id],dfn[id]+sz[id]-1),'\n');
        }
    }
    return 0;
}

可以看得出来在 modify(u,v,x) 函数中我犯了两个错，我将这个函数贴出来：

inline void modify(int u,int v,const uint x){
    while(top[u]!=top[v]){
        if(d[top[v]]<d[top[v]])swap(u,v);
        tre.modify(dfn[top[u]],dfn[u],v);
        u=fa[top[u]];
    }if(dfn[u]>dfn[v])swap(u,v);
    tre.modify(dfn[u],dfn[v],x);
}

首先，我的深度比较 d[top[u]]<d[top[v]] 打成了 d[top[v]]<d[top[v]].

其次，在线段树传参我应该打成 tre.modify(dfn[top[u]],dfn[u],x)，但是我打成了 tre.modify(dfn[top[u]],dfn[u],v).

我的错误似乎都很致命，但是这是测试记录，居然只 RE 两组，并且其他都过了，莫名拿下 $80pts$，这真的是离谱，难以相信是省选题...