2020-05-21

## 1277. Count Square Submatrices with All Ones

### Question:

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.
``````

Constraints:

• `1 <= arr.length <= 300`
• `1 <= arr.length <= 300`
• `0 <= arr[i][j] <= 1`

### Solution:

Similar to LeetCode Question 221, we use dp to solve the problem. The difference is that we change from max to a counter.

``````class Solution {
public int countSquares(int[][] matrix) {
int counter = 0;

for(int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
if (matrix[i][j] == 1) {
if ((i == 0 || j == 0)) {
matrix[i][j] = 1;
} else {
matrix[i][j] = 1 + Math.min(Math.min(matrix[i][j-1], matrix[i-1][j]), matrix[i-1][j-1]);
}
counter += matrix[i][j];
}

}
}

return counter;
}
}
``````