Given a matrix of m x n elements (m rows, n columns), return all elements of the matrix in spiral order.

For example,

Given the following matrix:

[ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7, 8, 9 ]]

You should return [1,2,3,6,9,8,7,4,5].

思路：设置四个边界变量。仅仅要不越界即可

public class Solution {	public List
      
        spiralOrder(int[][] matrix) {		ArrayList
       
         al = new ArrayList
        
         ();		if (matrix.length == 0)			return al;		int x1 = 0;		int y1 = 0;		int x2 = matrix.length - 1;		int y2 = matrix[0].length - 1;		while (x1 <= x2 && y1 <= y2) {			// up row			for (int i = y1; i <= y2; ++i)				al.add(matrix[x1][i]);			// right column			for (int i = x1 + 1; i <= x2; ++i)				al.add(matrix[i][y2]);			// bottom row			for (int i = y2 - 1; x2 != x1&& i >= y1; --i)				al.add(matrix[x2][i]);			// left column			for (int i = x2 - 1; y1 != y2 && i > x1; --i)				al.add(matrix[i][y1]);			x1++;			y1++;			x2--;			y2--;		}		return al;	}}
        
       
      

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