Leetcode_1277_Count Square Submatrices with All Ones

[lihe]

Count Square Submatrices with All Ones

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

class Solution{
    public int countSquares(int[][] matrix){
        int m = matrix.length;
        int n = matrix[0].length;
        int ans = 0;
        int[][] dp = new int[m][n];

        for(int i = 0; i < m; i++){
            ans += dp[i][0] = matrix[i][0];
        }
        for(int i = 0; i < n; i++){
            ans += dp[0][i] = matrix[0][i];
        }

        if(matrix[0][0] == 1){
            ans--;
        }

        for(int i = 1; i < m; i++){
            for(int j = 1; j < n; j++){
                if(matrix[i][j] == 1) {
                    dp[i][j] = Math.min(Math.min(dp[i - 1][j], dp[i][j - 1]), dp[i - 1][j - 1]) + 1;
                    ans += dp[i][j];
                }
            }
        }

        return ans;
    }
}