solution.java

class Solution {
    public int countSquares(int[][] matrix) {
        if (matrix == null || matrix.length == 0) return 0;
        
        int m = matrix.length;
        int n = matrix[0].length;
        int[][] dp = new int[m][n];
        int count = 0;
        
        // Fill first row
        for (int j = 0; j < n; j++) {
            dp[0][j] = matrix[0][j];
            count += dp[0][j];
        }
        
        // Fill first column
        for (int i = 1; i < m; i++) {
            dp[i][0] = matrix[i][0];
            count += dp[i][0];
        }
        
        // Fill rest of the dp table
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (matrix[i][j] == 1) {
                    // dp[i][j] = min of left, top, and top-left + 1
                    dp[i][j] = Math.min(dp[i-1][j], Math.min(dp[i][j-1], dp[i-1][j-1])) + 1;
                    count += dp[i][j];
                }
            }
        }
        
        return count;
    }
}