BZOJ 1196 二分+Kruskal

思路：

二分答案

判一下能不能加

//By SirisuRen
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int N=40050;
int n,m,k,xx,yy,aa,bb,ans,f[N],top;
struct Node{
    int from,to,type,wei;Node(){}
    Node(int FR,int TO,int TY,int WE){
        from=FR,to=TO,type=TY,wei=WE;
    }
}node[N];
bool cmp(Node a,Node b){return a.wei<b.wei;}
int find(int x){return x==f[x]?x:f[x]=find(f[x]);}
bool check(int mid){
    int cnt1=0;
    for(int i=1;i<=n;i++)f[i]=i;
    for(int i=1;i<=mid;i++){
        if(node[i].type!=1)continue;
        int fx=find(node[i].from),fy=find(node[i].to);
        if(fx!=fy)f[fx]=fy,cnt1++;
    }
    if(cnt1<k)return 0;
    for(int i=1;i<=mid;i++){
        if(node[i].type==1)continue;
        int fx=find(node[i].from),fy=find(node[i].to);
        if(fx!=fy)f[fx]=fy,cnt1++;
    }return cnt1==n-1;
}
int main(){
    scanf("%d%d%d",&n,&k,&m);
    for(int i=1;i<m;i++){
        scanf("%d%d%d%d",&xx,&yy,&aa,&bb);
        node[++top]=Node(xx,yy,1,aa);
        node[++top]=Node(xx,yy,2,bb);
    }
    sort(node+1,node+1+top,cmp);
    int l=n-1,r=top;
    while(l<=r){
        int mid=(l+r)>>1;
        if(check(mid))ans=mid,r=mid-1;
        else l=mid+1;
    }
    printf("%d\n",node[ans].wei);
}