Problem
Given a m * n matrix of ones and zeros, return how many square submatrices have all ones.
Example 1:
Input: matrix =
[
[0,1,1,1],
[1,1,1,1],
[0,1,1,1]
]
Output: 15
Explanation:
There are 10 squares of side 1.
There are 4 squares of side 2.
There is 1 square of side 3.
Total number of squares = 10 + 4 + 1 = 15.
Example 2:
Input: matrix =
[
[1,0,1],
[1,1,0],
[1,1,0]
]
Output: 7
Explanation:
There are 6 squares of side 1.
There is 1 square of side 2.
Total number of squares = 6 + 1 = 7.
Solution
DP problem. Same as p227.
class Solution:
def countSquares(self, matrix):
if not matrix or not matrix[0]:
return 0
m = len(matrix)
n = len(matrix[0])
ans = 0
for i in range(m):
for j in range(n):
if matrix[i][j] == 1:
if i and j:
matrix[i][j] = min(matrix[i-1][j-1], matrix[i][j-1], matrix[i-1][j]) + 1
ans += matrix[i][j]
return ans