436. 最大正方形

class Solution:
    """
    @param matrix: a matrix of 0 and 1
    @return: an integer
    """
    def maxSquare(self, matrix):
        # write your code here
        #经典的dp问题
        if not matrix: return 0
        
        #以当前点作为正方形的右下角,然后计算正方形的边长,滚动数组
        len_x, len_y = len(matrix), len(matrix[0])
        dp = [[0 for _ in range(len_y)], [0 for _ in range(len_y)]]
        result = 0 
        boder = 0 
        
        #边界情况
        for j in range(len_y):
            dp[0][j] = matrix[0][j]

        if len_x == 1:
            boder = max(matrix[0])
            
        #计算顺序
        for i in range(1, len_x):
            #边界情况
            dp[i%2][0] = matrix[i][0]
            for j in range(1, len_y):
                if (matrix[i][j] == 0):
                    dp[i%2][j] = 0
                    continue
                
                dp[i%2][j] = min(dp[(i - 1)%2][j], dp[i%2][j - 1], dp[(i - 1)%2][j - 1]) + 1
            boder = max(boder, max(dp[i%2]))
        
        return boder*boder