BZOJ 2756 奇怪的游戏（最大流）

struct node
{
    int v,next;
    i64 cap;
};




node edges[N];
int head[N],e;




void add(int u,int v,i64 cap)
{
    edges[e].v=v;
    edges[e].cap=cap;
    edges[e].next=head[u];
    head[u]=e++;
}




void Add(int u,int v,i64 cap)
{
    add(u,v,cap);
    add(v,u,0);
}




int pre[N],h[N],num[N],cur[N];




i64 Maxflow(int s,int t,int n)
{
    int i;
    for(i=0;i<=n;i++) h[i]=num[i]=0,cur[i]=head[i];
    int u=s,v,x;
    i64 ans=0,Min;
    
    while(h[u]<n)
    {
        if(u==t)
        {
            Min=inf+1;
            for(i=s;i!=t;i=edges[cur[i]].v)
            {
                x=cur[i];
                if(edges[x].cap<Min) Min=edges[x].cap,v=i;
            }
            ans+=Min; u=v;
            for(i=s;i!=t;i=edges[cur[i]].v)
            {
                x=cur[i];
                edges[x].cap-=Min;
                edges[x^1].cap+=Min;
            }
        }
        for(i=cur[u];i!=-1;i=edges[i].next)
        {
            v=edges[i].v;
            if(edges[i].cap>0&&h[u]==h[v]+1) break;
        }
        if(i!=-1)
        {
            cur[u]=i;
            pre[edges[i].v]=u;
            u=edges[i].v;
        }
        else
        {
            if(--num[h[u]]==0) break;
            cur[u]=head[u];
            x=n;
            for(i=head[u];i!=-1;i=edges[i].next)
            {
                if(edges[i].cap>0) x=min(x,h[edges[i].v]);
            }
            h[u]=x+1;
            num[x+1]++;
            if(u!=s) u=pre[u];
        }
    }
    return ans;
}


i64 a[55][55];
int n,m,b[55][55],s,t;
int dx[]={0,0,1,-1};
int dy[]={1,-1,0,0};


int check(i64 p)
{
    clr(head,-1); e=0;
    s=0; t=n*m+1;
    int i,j,k,x,y;
    i64 sum=0;
    FOR1(i,n) FOR1(j,m) 
    {
        if(p<a[i][j]) return 0;
        if((i+j)&1) Add(s,b[i][j],p-a[i][j]),sum+=p-a[i][j];
        else Add(b[i][j],t,p-a[i][j]);
        if((i+j)%2==0) continue;
        FOR0(k,4)
        {
            x=i+dx[k];
            y=j+dy[k];
            if(x>=1&&x<=n&&y>=1&&y<=m) 
            {
               Add(b[i][j],b[x][y],inf);
            }
        }
    }
    return Maxflow(s,t,t+1)==sum;
}


i64 cal()
{
    i64 s1=0,s2=0,cnt1=0,cnt2=0,Max=0,ans;
    int i,j,k=0;
    FOR1(i,n) FOR1(j,m) b[i][j]=++k;
    FOR1(i,n) FOR1(j,m)
    {
        upMax(Max,a[i][j]);
        if((i+j)%2==0) s1+=a[i][j],cnt1++;
        else s2+=a[i][j],cnt2++;
    }
    if(cnt1!=cnt2)
    {
        ans=(s1-s2)/(cnt1-cnt2);
        if(check(ans)) return ans*cnt1-s1;
        return -1;
    }
    
    
    if(s1!=s2) return -1;
    
    i64 low=Max,high=inf,mid;
    ans=-1;
    while(low<=high)
    {
        mid=(low+high)>>1;
        if(check(mid)) ans=mid,high=mid-1;
        else low=mid+1;
    }
    if(ans==-1) return -1;
    return ans*cnt1-s1;
}


int main()
{
    rush()
    {
        RD(n,m);
        int i,j;
        FOR1(i,n) FOR1(j,m) RD(a[i][j]);
        PR(cal());
    }
}