hdu4328 最大子矩陣問題O（n*m）掃描思想

http://acm.hdu.edu.cn/showproblem.php?pid=4328

Mark bought a huge cake, because his friend ray_sun’s birthday is coming. Mark is worried about how to divide the cake since it’s so huge and ray_sun is so strange. Ray_sun is a nut, you can never imagine how strange he was, is, and going to be. He does not eat rice, moves like a cat, sleeps during work and plays games when the rest of the world are sleeping……It is not a surprise when he has some special requirements for the cake. A considering guy as Mark is, he will never let ray_sun down. However, he does have trouble fulfilling ray_sun’s wish this time; could you please give him a hand by solving the following problem for him?
  The cake can be divided into n*m blocks. Each block is colored either in blue or red. Ray_sun will only eat a piece (consisting of several blocks) with special shape and color. First, the shape of the piece should be a rectangle. Second, the color of blocks in the piece should be the same or red-and-blue crisscross. The so called ‘red-and-blue crisscross’ is demonstrated in the following picture. Could you please help Mark to find out the piece with maximum perimeter that satisfies ray_sun’s requirements?

/**
hdu4328 最大子矩陣問題O（n*m）
題目大意：給定一個n*m的棋盤有紅黑兩色，讓擷取一個周長最大矩形，該矩形要麼全是黑色，要麼全是紅色，要麼黑色和紅色交替
           見題目附圖。
解題思路：如果遍歷每一個矩陣是不可能的，時間O（n*n*n），因此我們採用掃描的方法（白書P51）。對於交替的矩形我們把（i+j）
           為奇數的格子翻轉顏色，那樣就可以轉換成全是一樣顏色了，然後邊成把（i+j）為偶數的翻轉，四種情況取最大即可。複雜度O（n*m）
*/
#include <stdio.h>
#include <string.h>
#include <algorithm>
#include <iostream>
using namespace std;

const int maxn=1005;
int mat[maxn][maxn],mat1[maxn][maxn],up[maxn][maxn],lef[maxn][maxn],rig[maxn][maxn];
char a[maxn][maxn];
int n,m;
int get()
{
    int ans=0;
    for(int i=0; i<m; i++)
    {
        int lo=-1,ro=n;
        for(int j=0; j<n; j++)
        {
            if(mat[i][j]==1)
            {
                up[i][j]=lef[i][j]=0;
                lo=j;
            }
            else
            {
                up[i][j]=i==0?1:up[i-1][j]+1;
                lef[i][j]=i==0?lo+1:max(lef[i-1][j],lo+1);
            }
        }
        for(int j=n-1; j>=0; j--)
        {
            if(mat[i][j]==1)
            {
                rig[i][j]=n;
                ro=j;
            }
            else
            {
                rig[i][j]=i==0?ro-1:min(rig[i-1][j],ro-1);
                ans=max(ans,up[i][j]*2+2*(rig[i][j]-lef[i][j]+1));
            }
        }
    }
    return ans;
}
int main()
{
    int tt=0,T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d",&m,&n);
        for(int i=0; i<m; i++)
        {
            scanf("%s",a[i]);
        }
        for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                if(a[i][j]=='R')
                    mat1[i][j]=mat[i][j]=1;
                else
                    mat1[i][j]=mat[i][j]=0;
            }
        }
        int ans=-1;
        ///1
        ans=max(ans,get());
        ///2
        for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                if(mat1[i][j]==1)
                    mat[i][j]=0;
                else
                    mat[i][j]=1;
            }
        }
        ans=max(ans,get());
        ///3
        for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                if((i+j)%2==1)
                {
                    if(mat1[i][j]==1)
                        mat[i][j]=0;
                    else
                        mat[i][j]=1;
                }
                else
                    mat[i][j]=mat1[i][j];
            }
        }
       /* for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                printf("%d",mat[i][j]);
            }
            printf("\n");
        }
        printf("\n");*/
        ans=max(ans,get());
        ///4
        for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                if((i+j)%2==0)
                {
                    if(mat1[i][j]==1)
                        mat[i][j]=0;
                    else
                        mat[i][j]=1;
                }
                else
                    mat[i][j]=mat1[i][j];
            }
        }
        /*for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                printf("%d",mat[i][j]);
            }
            printf("\n");
        }
        printf("\n");*/
        ans=max(ans,get());
        printf("Case #%d: %d\n",++tt,ans);
    }
    return 0;
}
/**
100
3 3
BBR
RBR
BBB

2 2
BR
RB

2 2
BB
RR

2 2
BB
BB
*/