题目链接：http://www.lydsy.com/JudgeOnline/problem.php?id=1196
题目大意：某城市要修建n-1条路使n个城市连通，要求n-1条路中必须有k条为高级公路，求花费最多的一条公路最少为多少

使最大值最小是二分的典型问题，直接二分最大值，然后跑最小生成树判断是否能够修建即可

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
struct Link
{
    int s,t,c1,c2;
}l[50000];
int n,m,k;
int fa[20000];

int father(int x)
{
    if(fa[x] != x)
        fa[x] = father(fa[x]);
    return fa[x];
}

bool same(int x,int y)
{
    return father(x) == father(y);
}

void add(int x,int y)
{
    fa[father(x)] = father(y);
    return ;
}

bool cmp1(Link x,Link y)
{
    return x.c1 < y.c1;
}

bool cmp2(Link x,Link y)
{
    return x.c2 < y.c2;
}

bool judge(int x)
{
    sort(l+1,l+m+1,cmp1);
    int p = 0;
    for(int i = 1;p < k;i ++)
    {
        if(l[i].c1 > x || i > m)
            return 0;
        if(!same(l[i].s,l[i].t))
        {
            add(l[i].s,l[i].t);
            p ++;
        }
    }
    sort(l+1,l+m+1,cmp2);
    p = 0;
    for(int i = 1;p < n-k-1;i ++)
    {
        if(l[i].c2 > x || i > m)
            return 0;
        if(!same(l[i].s,l[i].t))
        {
            add(l[i].s,l[i].t);
            p ++;
        }
    }
    return 1;
}

int main()
{
    int s,t,c1,c2;
    scanf("%d%d%d",&n,&k,&m);
    for(int i = 1;i <= m;i ++)
    {
        scanf("%d%d%d%d",&s,&t,&c1,&c2);
        l[i].s = s;
        l[i].t = t;
        l[i].c1 = c1;
        l[i].c2 = c2;
    }
    int l = 0,r = 50000;
    while(l != r)
    {
        for(int i = 1;i <= n;i ++)
            fa[i] = i;
        int mid = (l + r) / 2;
        if(judge(mid))
            r = mid;
        else
            l = mid + 1;
    }
    cout << l << endl;
}