19 顺时针打印矩阵

输入一个矩阵，按照从外向里以顺时针的顺序依次打印出每一个数字，例如，如果输入如下4 X 4矩阵： 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 则依次打印出数字1,2,3,4,8,12,16,15,14,13,9,5,6,7,11,10.

import java.util.ArrayList;
public class Solution {
    public ArrayList<Integer> printMatrix(int [][] matrix) {
        ArrayList<Integer> list = new ArrayList<>();
        if(matrix == null || matrix.length == 0 || matrix[0] ==null || matrix[0].length == 0){
            return list;
        }
        int rows = matrix.length, cols = matrix[0].length, start = 0;
        while(rows > start * 2 && cols > start * 2){
            printOneCycle(matrix, start, list);
            start++;
        }
        return list;
    }

    public void printOneCycle(int[][] matrix, int start, ArrayList<Integer> list){
        int endX = matrix[0].length - 1 - start;
        int endY = matrix.length - 1 - start;

        //从左往右
        for(int i = start; i <= endX; i++){
            list.add(matrix[start][i]);
        }
        //从上往下
        if(start < endY){
            for(int i = start + 1; i <= endY; i++){
                list.add(matrix[i][endX]);
            }
        }
        //从右往左
        if(start < endX & start < endY){
            for(int i = endX - 1; i >= start; i--){
                list.add(matrix[endY][i]);
            }
        }
        //从下往上
        if(start < endX && start < endY - 1){
            for(int i = endY - 1; i >= start + 1; i--){
                list.add(matrix[i][start]);
            }
        }
    }
}