解题思路：

设置四个边界

class Solution {public List<Integer> spiralOrder(int[][] matrix) {if (matrix.length == 0) return new ArrayList<Integer>();int l = 0;int r = matrix[0].length - 1;int t = 0;int b = matrix.length - 1;List<Integer> res = new ArrayList<>();while (true) {for (int i = l; i <= r; i++) res.add(matrix[t][i]);if (++t > b) break;for (int i = t; i <= b; i++) res.add(matrix[i][r]);if (--r < l) break;for (int i = r; i >= l; i--) res.add(matrix[b][i]);if (--b < t) break;for (int i = b; i >= t; i--) res.add(matrix[i][l]);if (++l > r) break;}return res;}
}