[codeforces 852 D] Exploration Plan 解题报告 （二分+最大匹配）

#include<algorithm>
#include<cstring>
#include<iostream>
#include<cstdio>
using namespace std;

const int N=600+15;
const int inf=1e9+7;
const int Inf=1731311;
int v,e,n,k;
int pos[N],d[N][N],mp[N][N],match[N],vis[N];
inline int read()
{
    char ch=getchar();
    int s=0,f=1;
    while (ch<‘0‘||ch>‘9‘) {if (ch==‘-‘) f=-1;ch=getchar();}
    while (ch>=‘0‘&&ch<=‘9‘) {s=(s<<3)+(s<<1)+ch-‘0‘;ch=getchar();}
    return s*f;
}
void floyd()
{
    for (int k=1;k<=v;k++)
        for (int i=1;i<=v;i++)    
            for (int j=1;j<=v;j++) d[i][j]=min(d[i][j],d[i][k]+d[k][j]);
}
bool dfs(int x)
{
    for (int i=1;i<=v;i++)
    {
        if (vis[i]||!mp[x][i]) continue;
        vis[i]=1;
        if (match[i]==-1||dfs(match[i]))
        {
            match[i]=x;
            return true;        
        }
    }
    return false;
}
int check(int l)
{
    memset(match,-1,sizeof(match));
    for (int i=1;i<=n;i++)    
        for (int j=1;j<=v;j++)
         mp[i][j]=(d[pos[i]][j]<=l);
    int ans=0;
    for (int i=1;i<=n;i++)
    {
        memset(vis,0,sizeof(vis));
        if (dfs(i)) ans++;
    }
    return ans;    
}
int main()
{
    v=read();e=read();n=read();k=read();
    for (int i=1;i<=v;i++)    
        for (int j=1;j<=v;j++) if (i!=j) d[i][j]=inf;
    for (int i=1;i<=n;i++) pos[i]=read();
    for (int i=1,u,v,w;i<=e;i++)
    {
        u=read();v=read();w=read();
        d[u][v]=d[v][u]=min(d[u][v],w);
    }
    floyd();
    int l=0,r=Inf,ans=Inf;
    while (l<r)
    {
        int mid=l+r>>1;
        if (check(mid)>=k) r=mid,ans=min(ans,r);
        else l=mid+1;
    }
    if (ans==Inf) puts("-1");
    else printf("%d\n",ans);
    return 0;
}