```
class Solution(object):
def maximalSquare(self, matrix):
"""
:type matrix: List[List[str]]
:rtype: int
"""
matrix = [[int(x) for x in y] for y in matrix]
if not matrix or not matrix[0]: return 0
m, n = len(matrix), len(matrix[0])
for i in range(m-2, -1, -1):
for j in range(n-2, -1, -1):
if matrix[i][j] == 0: continue
min_size = min(matrix[i][j+1], matrix[i+1][j], matrix[i+1][j+1])
if min_size>0 : matrix[i][j] = 1+min_size
return max(max(x) for x in matrix)**2
```