51Nod1640：晴朗天气下的神奇魔法现象分析

#include 
#include 
#include 
#include 
#define N 100005
typedef long long LL;
inline void Read(LL &x)
{
    bool f=0;register char ch=getchar();
    for(x=0;!isdigit(ch);ch=getchar()) if(ch=='-') f=1;
    for(;isdigit(ch);x=x*10+ch-'0',ch=getchar());
    x=f?-x:x;
}
using namespace std;
struct node
{
    LL x,y,z;
    bool operator<(node a)const
    {
        return z<a.z;
    }
}edge[N<<1];
LL fa[N],n,m;
LL ans=-0x7fffffff,l,r;
LL find_(LL x) {return fa[x]==x?x:fa[x]=find_(fa[x]);}
LL max(LL a,LL b) {return a>b?a:b;}
LL min(LL a,LL b) {return a>b?b:a;}
bool check(LL x)
{
    for(LL i=1;i<=n;++i) fa[i]=i;
    LL now=0,num=0;
    for(LL i=m;i>=1;--i)
    {
        int fx=find_(edge[i].x),fy=find_(edge[i].y);
        if(fx!=fy&&edge[i].z<=x)
        {
            fa[fy]=fx;
            now+=edge[i].z;
            num++;
            if(num==n-1) break;
        }
    }
    if(num==n-1) {ans=now;return true;} 
    return false;
}
int main()
{
    Read(n);Read(m);
    for(int i=1;i<=m;++i) Read(edge[i].x),Read(edge[i].y),Read(edge[i].z);
    l=-0x7fffffff,r=0x7fffffff;
    sort(edge+1,edge+1+m);
    for(LL mid;l<=r;)
    {
        mid=(l+r)>>1;
        if(check(mid)) r=mid-1;
        else l=mid+1;
    }
    printf("%lld\n",ans);
    return 0;
}