计蒜客 T2237 魔法 分类讨论

Code:

#include<bits/stdc++.h>
#define setIO(s) freopen(s".in","r",stdin)
#define maxn 200005
using namespace std;
int du[maxn],from[maxn],to[maxn];
int main()
{
    // setIO("input");
    int n,k;
    scanf("%d%d",&n,&k);
    for(int i=1;i<n;++i)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        ++du[x],++du[y];
        from[i]=x, to[i]=y;
    }
    int ans=0, cnt=0;
    for(int i=1;i<=n;++i) if(du[i]==1) ++ans;
    if(k==0) printf("%d\n",ans);
    else
    {
        cnt=ans;
        for(int i=1;i<n;++i)
        {
            int u=from[i], v=to[i];
            int x=min(du[u], du[v]);
            int y=max(du[u], du[v]);
            if(x<=2&&y>=3) ans=min(ans, cnt-1);
            if(x>=3) ans=min(ans, cnt-2);
        }
        printf("%d\n",ans);
    }
    return 0;
}