[online programming] Print matrix clockwise

[problem description]
Enter a matrix and print out each number in a clockwise order from the outside to the inside. For example, if you enter the following 4 X 4 matrix: 1 23 4 5 6 7 8 9 10 11 12 13 14 15 16, then print out the numbers 1,2,3,4,8,12,16,15,14,13,9,5,6,7,11,10

[Solution & Java implementation]

Idea: first print the periphery (pay attention to row by column), and then shrink the circle.
For a two-dimensional array, row pointer i and column pointer j are required to locate an element, and row1,row2,col1,col2 are required to locate a two-dimensional array.

import java.util.ArrayList;

public class Solution {
    public ArrayList<Integer> printMatrix(int[][] matrix) {
        int row1 = 0;
        int row2 = matrix.length - 1;
        int col1 = 0;
        int col2 = matrix[0].length - 1;
        ArrayList<Integer> result = new ArrayList<>();
        
        //Shrinking circle
        while (row1 <= row2 && col1 <= col2) {
            result.addAll(printBoundary(matrix, row1, row2, col1, col2));
            row1++;
            row2--;
            col1++;
            col2--;
        }
        return result;
 
    }
    
    /**
    *Print a circle
    */
    public ArrayList<Integer> printBoundary(int[][] matrix, int row1, int row2, int col1, int col2) {
        ArrayList<Integer> list = new ArrayList<>();
        //If it's a line
        if (row1 == row2) {
            for (int j = col1; j <= col2; j++) {
                list.add(matrix[row1][j]);
            }
        } else if (col1 == col2) {
            for (int i = row1; i <= row2; i++) {
                list.add(matrix[i][col2]);
            }
        } else {
            for (int j = col1; j <= col2; j++) {
                list.add(matrix[row1][j]);
            }
 
            for (int i = row1 + 1; i <= row2; i++) {
                list.add(matrix[i][col2]);
            }
 
            for (int j = col2 - 1; j >= col1; j--) {
                list.add(matrix[row2][j]);
            }
 
            for (int i = row2 - 1; i > col1; i--) {
                list.add(matrix[i][col1]);
            }
        }
 
        return list;
    }
}