Soyo

May Leetcode Challenge - Day 21

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

Solution

DP with formula f(i,j) = min(f(i-1,j), f(i,j-1), f(i-1,j-1))

class Solution {
    func countSquares(_ matrix: [[Int]]) -> Int {
        var copy = matrix
        var count: Int = 0
        for i in 0..<copy.count {
            for j in 0..<copy[0].count {
                if (i != 0 && j != 0 && matrix[i][j] != 0) {
                    copy[i][j] = min(copy[i-1][j], min(copy[i][j-1], copy[i-1][j-1])) + 1
                }
                count += copy[i][j]
            }
        }
        
        return count
    }
}

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