BZOJ 4443 小凸玩矩阵

二分+网络流。

#include<iostream>
#include<cstring>
#include<cstdio>
#include<queue>
#include<algorithm>
#define maxn 255
#define maxv 10050
#define maxe 500500
#define inf 1000000007
using namespace std;
int n,m,k,g[maxv],nume=1,map[maxn][maxn],s,t;
int dis[maxv];
bool vis[maxv];
queue <int> q; 
struct edge
{
    int v,f,nxt;
}e[maxe];
void addedge(int u,int v,int f)
{
    e[++nume].v=v;e[nume].f=f;e[nume].nxt=g[u];g[u]=nume;
    e[++nume].v=u;e[nume].f=0;e[nume].nxt=g[v];g[v]=nume;
}
bool bfs()
{
    while (!q.empty()) q.pop();
    for (int i=s;i<=t;i++) {dis[i]=inf;vis[i]=false;}
    dis[s]=0;vis[s]=true;q.push(s);
    while (!q.empty())
    {
        int head=q.front();q.pop();
        for (int i=g[head];i;i=e[i].nxt)
        {
            int v=e[i].v;
            if ((!vis[v]) && (e[i].f))
            {
                vis[v]=true;dis[v]=dis[head]+1;
                q.push(v);
            }
        }
    }
    if (!vis[t]) return false;
    return true;
}
int dinic(int x,int low)
{
    if (x==t) return low;
    int ret=0;
    for (int i=g[x];i && low;i=e[i].nxt)
    {
        int v=e[i].v;
        if ((dis[v]==dis[x]+1) && (e[i].f))
        {
            int dd=dinic(v,min(low,e[i].f));
            e[i].f-=dd;e[i^1].f+=dd;
            low-=dd;ret+=dd;
        }
    }
    if (!ret) dis[x]=inf;
    return ret;
}
bool check(int x)
{
    memset(g,0,sizeof(g));
    nume=1;
    for (int i=1;i<=n;i++) addedge(s,i,1);
    for (int i=1;i<=m;i++) addedge(i+n,t,1);
    for (int i=1;i<=n;i++)
        for (int j=1;j<=m;j++)
        {
            if (map[i][j]<=x)
                addedge(i,j+n,1);
        }
    int max_flow=0;
    while (bfs())
        max_flow+=dinic(s,inf);
    if (max_flow<n-k+1) return false;
    return true;
}
int half_search()
{
    int l=1,r=1000000000,ans;
    while (l<=r)
    {
        int mid=l+r>>1;
        if (check(mid)) {ans=mid;r=mid-1;}
        else l=mid+1;
    }
    return ans;
}
int main()
{
    scanf("%d%d%d",&n,&m,&k);
    s=0;t=n+m+1;
    for (int i=1;i<=n;i++)
        for (int j=1;j<=m;j++)
            scanf("%d",&map[i][j]);
    printf("%d
",half_search());
    return 0;
}