HDU_4912 Path on the tree 2014多校5 贪心+LCA

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <queue>
using namespace std;
const int N = (100000+10)*2;
int u[N],v[N],ft[N],nt[N],cnt;
int dep[N];
vector<int> G[N];
void add(int a,int b)
{
    u[cnt]=a;
    v[cnt]=b;
    nt[cnt]=ft[a];
    ft[a]=cnt++;
}
int f[N];
int anc[N];
int vis[N],col[N];
struct edge
{
    int a,b,anc,dep;
    bool operator < (const edge& rhs) const{
        return dep>rhs.dep;
    }
}E[N];
int findset(int x)
{
    if (x!=f[x]){
       f[x]=findset(f[x]);
    }
    return f[x];
}
int unionset(int a,int b)
{
    int r1=findset(a);
    int r2=findset(b);
    if (r1==r2) return r1;
    f[r2]=r1;
    return r1;
}
void tarjan(int x,int fa)
{
    anc[x]=x;
    for (int i=ft[x];i!=-1;i=nt[i]){
        int nx=v[i];
        if (nx==fa) continue;
        tarjan(nx,x);
        int rt=unionset(x,nx);
        anc[rt]=x;
    }
    col[x]=1;
    for (int i=0;i<G[x].size();i++){
        int ii=G[x][i];
        int nx=E[ii].a^E[ii].b^x;
        if (col[nx]){
            E[ii].anc=findset(nx);
            E[ii].dep=dep[E[ii].anc];

        }
    }
}
void dfs1(int x,int fa,int d)
{
    dep[x]=d;
    for (int i=ft[x];i!=-1;i=nt[i]){
        int vx=v[i];
        if (vx==fa) continue;
        dfs1(vx,x,d+1);
    }
}
void dfs2(int x)
{
    vis[x]=1;
    for (int i=ft[x];i!=-1;i=nt[i]){
        int vx=v[i];
        if (dep[vx]<dep[x] || vis[vx]) continue;
        dfs2(vx);
    }
}
int main()
{
    int n,m,a,b;
    while (scanf("%d%d",&n,&m)!=EOF)
    {
        memset(ft,-1,sizeof(ft));
        memset(vis,0,sizeof(vis));
        memset(col,0,sizeof(col));
        cnt=0;
        for (int i=1;i<n;i++){
            scanf("%d%d",&a,&b);
            add(a,b);
            add(b,a);
            G[i].clear();
            f[i]=i;
        }
        dfs1(1,-1,0);
        G[n].clear();
        f[n]=n;
        for (int i=0;i<m;i++){
            scanf("%d%d",&a,&b);
            G[a].push_back(i);
            G[b].push_back(i);
            E[i].a=a;
            E[i].b=b;
        }
        tarjan(1,-1);
        sort(E,E+m);
        int ans=0;
        for (int i=0;i<m;i++){
            if (!vis[E[i].a] && !vis[E[i].b]){
                ans++;
                dfs2(E[i].anc);
            }
        }
        printf("%d\n",ans);
    }
    return 0;
}