leetcode 85. Maximal Rectangle

 

Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing all ones and return its area.

 

class Solution {
public:

int largestRectangleArea(vector<int>& heights) {
        int n = heights.size();
        int i,j;
        int max=0;
        stack<int> sk;
        vector<int> down;
        sk.push(0);
        if(n== 0)
        return 0;
        for(i=0;i<n;i++)
        {
            down.push_back(0);
        }
        for(i=1;i<n;i++)
        {
            while(!sk.empty())
            {
                int t =sk.top();
                if(heights[i]>=heights[t])
                {
                    break;
                }
                else
                {
                    int area = heights[t]*(i-t+down[t]);
                    if(area > max)
                    max = area;
                    sk.pop();
                    down[i] = down[i] +1 + down[t];
                    
                }
            }
            sk.push(i);
            
        }
        while(!sk.empty())
        {
            int t = sk.top();
            int area = heights[t]*(n-t+down[t]);
            if(area > max)
            max = area;
            sk.pop();
        }
        return max;
    }
    int maximalRectangle(vector< vector<char> >& matrix) {
        vector<vector<int>> maxlen;
        int m = matrix.size();
        if(m==0)
        return 0;
        int n = matrix[0].size();
        if(n==0)
        return 0;
        int i,j;
        int max=0;
        maxlen.resize(m+1);
        for(i=0;i<m;i++)
        {
            maxlen[i].resize(n+1);
            for(j=n-1;j>=0;j--)
            {
                if(j==n-1)
                {
                     if(matrix[i][j]=='0')
                     maxlen[i][j]=0;
                     else
                     maxlen[i][j]=1;
                }
                else
                {
                    if(matrix[i][j]=='0')
                    maxlen[i][j]=0;
                    else
                    {
                        maxlen[i][j] = maxlen[i][j+1]+1;
                    }
                }
            }
        }
        for(j=0;j<n;j++)
        {
            int start = 0;
            while(start<m)
            {
                vector<int> heights;
                while(start<m &&maxlen[start][j]==0)start++;
                while(start<m &&maxlen[start][j] !=0)
                {
                    heights.push_back(maxlen[start][j]);
                    start++;
                }
                if(heights.size()!=0)
                {
                    int ar=largestRectangleArea(heights);
                    if(ar>max)
                    max=ar;
                }
            }
        }
        return max;
      
    }
};