maximal-rectangle.py

# Time:  O(n^2)
# Space: O(n)
#
# 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:
    # @param matrix, a list of lists of 1 length string
    # @return an integer
    def maximalRectangle(self, matrix):
        if not matrix:
            return 0
        
        result = 0
        m = len(matrix)
        n = len(matrix[0])
        L = [0 for _ in xrange(n)]
        H = [0 for _ in xrange(n)]
        R = [n for _ in xrange(n)]

        for i in xrange(m):
            left = 0
            for j in xrange(n):
                if matrix[i][j] == '1':
                    L[j] = max(L[j], left)
                    H[j] += 1
                else:
                    L[j] = 0
                    H[j] = 0
                    R[j] = n
                    left = j + 1
                    
            right = n
            for j in reversed(xrange(n)):
                if matrix[i][j] == '1':
                    R[j] = min(R[j], right)
                    result = max(result, H[j] * (R[j] - L[j]))
                else:
                    right = j
                    
        return result

if __name__ == "__main__":
    matrix = ["01101",
              "11010",
              "01110",
              "11110",
              "11111",
              "00000"]
    print Solution().maximalRectangle(matrix)