代码：

class Solution {public List<Integer> spiralOrder(int[][] matrix) {List<Integer> res = new ArrayList<>();if(matrix==null||matrix.length==0||matrix[0].length==0)return res;int left=0,right=matrix[0].length-1,up=0,down = matrix.length-1;while(left<=right&&up<=down){for(int i=left;i<=right;i++){res.add(matrix[up][i]);}for(int i=up+1;i<=down;i++){res.add(matrix[i][right]);}for(int i=right-1;i>=left&&up!=down;i--){res.add(matrix[down][i]);}for(int i=down-1;i>up&&left!=right;i--){res.add(matrix[i][left]);}left++;right--;up++;down--;}return res;}
}