rt，我写的就是现贪心选尽量大的子树然后再构造方案，通过代码如下：

#include<bits/stdc++.h>
#define ll long long
#define N 500005
using namespace std;
ll read(){
    ll x=0,f=1;char ch=getchar();
    while(ch<'0' || ch>'9')f=(ch=='-'?-1:f),ch=getchar();
    while(ch>='0' && ch<='9')x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    return x*f;
}
void write(ll x){
    if(x<0)x=-x,putchar('-');
    if(x/10)write(x/10);
    putchar(x%10+'0');
}
vector<ll> e[N];
ll siz[N];
struct Node{
    ll val,num;
    Node(ll val=0,ll num=0):val(val),num(num){}
}a[N];
bool cmp(Node x,Node y){
    return x.val>y.val;
}
void dfs(ll x,ll fa){
    siz[x]=1;
    for(auto v:e[x])if(v!=fa)dfs(v,x),siz[x]+=siz[v];
}
ll act[N],cnt,tot;
void dfs2(ll x,ll fa){
    if(act[x])act[x]=++cnt;
    for(auto v:e[x])if(v!=fa)dfs2(v,x);
    if(!act[x])act[x]=++tot;
}
int main(){
    #ifdef EAST_CLOUD
    freopen("a.in","r",stdin);
    freopen("a.out","w",stdout);
    #endif
    ll n=read(),k=read();
    for(ll i=1;i<n;i++){
        ll u=read(),v=read();
        e[u].push_back(v);e[v].push_back(u);
    }
    dfs(1,0);ll sum=0;
    for(ll i=1;i<=n;i++)sum+=siz[i],a[i]=Node(siz[i],i);
    if(k>sum || k<n){printf("No\n");return 0;}
    sort(a+1,a+n+1,cmp);
    for(ll i=1;i<=n;i++){
        if(k>=a[i].val)k-=a[i].val,cnt++,act[a[i].num]=1;
    }
    cnt=n-cnt;
    dfs2(1,0);printf("Yes\n");
    for(ll i=1;i<=n;i++)write(act[i]),putchar(' ');
    return 0;
}

而我一开始将其中的 dfs2 函数写成了

void dfs2(ll x,ll fa){
    if(act[x])act[x]=++cnt;
    if(!act[x])act[x]=++tot;
    for(auto v:e[x])if(v!=fa)dfs2(v,x);
}

即将对没被选择的点按上升顺序标号，但是我觉得这应该没有问题，因为根节点必然被选择且标号比所有没被选择的点标号都大，因此无论顺序怎样必然不影响答案，但是这份代码却 WA 了，想问下是为什么。

WA 完整代码：

#include<bits/stdc++.h>
#define ll long long
#define N 500005
using namespace std;
ll read(){
    ll x=0,f=1;char ch=getchar();
    while(ch<'0' || ch>'9')f=(ch=='-'?-1:f),ch=getchar();
    while(ch>='0' && ch<='9')x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    return x*f;
}
void write(ll x){
    if(x<0)x=-x,putchar('-');
    if(x/10)write(x/10);
    putchar(x%10+'0');
}
vector<ll> e[N];
ll siz[N];
struct Node{
    ll val,num;
    Node(ll val=0,ll num=0):val(val),num(num){}
}a[N];
bool cmp(Node x,Node y){
    return x.val>y.val;
}
void dfs(ll x,ll fa){
    siz[x]=1;
    for(auto v:e[x])if(v!=fa)dfs(v,x),siz[x]+=siz[v];
}
ll act[N],cnt,tot;
void dfs2(ll x,ll fa){
    if(act[x])act[x]=++cnt;
    else act[x]=++tot;
    for(auto v:e[x])if(v!=fa)dfs2(v,x);
}
int main(){
    #ifdef EAST_CLOUD
    freopen("a.in","r",stdin);
    freopen("a.out","w",stdout);
    #endif
    ll n=read(),k=read();
    for(ll i=1;i<n;i++){
        ll u=read(),v=read();
        e[u].push_back(v);e[v].push_back(u);
    }
    dfs(1,0);ll sum=0;
    for(ll i=1;i<=n;i++)sum+=siz[i],a[i]=Node(siz[i],i);
    if(k>sum || k<n){printf("No\n");return 0;}
    sort(a+1,a+n+1,cmp);
    for(ll i=1;i<=n;i++){
        if(k>=a[i].val)k-=a[i].val,cnt++,act[a[i].num]=1;
    }
    cnt=n-cnt;
    dfs2(1,0);printf("Yes\n");
    for(ll i=1;i<=n;i++)write(act[i]),putchar(' ');
    return 0;
}