luogu 1169 [ZJOI2007]棋盘制作 悬线dp

悬线法，虽然得不到局部最优解，但是一定能得到全局最优解的算法，十分神奇~

#include <cstdio>
#include <algorithm>
#define N 2003
#define setIO(s) freopen(s".in","r",stdin)
using namespace std;
int n,m,ans1,ans2;
int v[N][N],l[N][N],r[N][N],up[N][N],len[N][N];
int main()
{
    int i,j;
    // setIO("input");
    scanf("%d%d",&n,&m);
    for(i=1;i<=n;++i)
    {
        for(j=1;j<=m;++j)
            scanf("%d",&v[i][j]),l[i][j]=r[i][j]=j, up[i][j]=1,len[i][j]=1;
    }
    for(i=1;i<=n;++i)
    {
        for(j=2;j<=m;++j) if(v[i][j]!=v[i][j-1]) l[i][j]=l[i][j-1];
        for(j=m-1;j>=1;--j) if(v[i][j]!=v[i][j+1]) r[i][j]=r[i][j+1];
    }
    for(i=1;i<=n;++i)
    {
        for(j=1;j<=m;++j)
        {
            if(i>1 && v[i][j]!=v[i-1][j])
            {
                l[i][j]=max(l[i][j], l[i-1][j]);
                r[i][j]=min(r[i][j], r[i-1][j]);
                up[i][j]=up[i-1][j]+1;
            }
            ans1=max(ans1, up[i][j]*(r[i][j]-l[i][j]+1));
        }
    }
    for(i=1;i<=n;++i)
    {
        for(j=1;j<=m;++j)
        {
            if(i>1&&j>1&&v[i][j]!=v[i][j-1]&&v[i][j]!=v[i-1][j]&&v[i][j]==v[i-1][j-1])
            {
                len[i][j]=min(len[i][j-1], min(len[i-1][j], len[i-1][j-1]))+1;  
 
            }
        }
    }
    for(i=1;i<=n;++i)
    {
        for(j=1;j<=m;++j) ans2=max(ans2, len[i][j]*len[i][j]);
    }
    printf("%d\n%d\n",ans2,ans1);
    return 0;
}