BZOJ 1475 方格取数

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#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<queue>
#define maxn 35
#define maxv 1050
#define maxe 500500
#define inf 1000000007
using namespace std;
int n,a[maxn][maxn],g[maxv],nume=1,sum=0,max_flow=0,dis[maxv],s,t;
int dx[]={0,1,0,-1,0},dy[]={0,0,1,0,-1};
struct edge
{
    int v,f,nxt;
}e[maxe];
queue <int> q;
int f(int x,int y) {return (x-1)*n+y;}
bool judge(int x,int y)
{
    return ((x>=1) && (x<=n) && (y>=1) && (y<=n));
}
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;
    dis[s]=0;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 (e[i].f<=0) continue;
            if (dis[v]>dis[head]+1)
            {
                dis[v]=dis[head]+1;
                q.push(v);
            }
        }
    }
    return dis[t]!=inf;
}
int dinic(int x,int low)
{
    if (x==t) return low;
    int ret=0;
    for (int i=g[x];low && i;i=e[i].nxt)
    {
        int v=e[i].v;
        if ((e[i].f>0) && (dis[v]==dis[x]+1))
        {
            int dd=dinic(v,min(low,e[i].f));
            e[i].f-=dd;e[i^1].f+=dd;
            ret+=dd;low-=dd;
        }
    }
    if (!ret) dis[x]=inf;
    return ret;
}
int main()
{
    scanf("%d",&n);s=0;t=n*n+1;
    for (int i=1;i<=n;i++)
        for (int j=1;j<=n;j++)
        {
            scanf("%d",&a[i][j]);sum+=a[i][j];
            if ((i+j)&1) addedge(s,f(i,j),a[i][j]);
            else addedge(f(i,j),t,a[i][j]);
        }
    for (int i=1;i<=n;i++)
        for (int j=1;j<=n;j++)
        {
            if (!((i+j)&1)) continue;
            for (int k=1;k<=4;k++)
            {
                int tx=i+dx[k],ty=j+dy[k];
                if (judge(tx,ty)) addedge(f(i,j),f(tx,ty),inf);
            }
        }
    while (bfs())
        max_flow+=dinic(s,inf);
    printf("%d\n",sum-max_flow);
    return 0;
}