示例 3：

输入：matrix = [["0"]]
输出：0

# 最大正方形面积
def max_square(matrix):
    m = len(matrix)
    n = len(matrix[0])
    if m == 0 or n == 0::
        return None
    max_side = 1
    dp = [[0] * (n + 1) for _ in range(m + 1)]
    for i in range(1, m + 1):
        for j in range(1, n + 1):
            if matrix[i][j] == 1:
                dp[i][j] = min(dp[i - 1][j - 1], dp[i - 1][j], dp[i][j - 1 ]) + 1
            max_side = max(max_side, dp[i][j])

    return max_side * max_side