看这个解析：
https://leetcode-cn.com/problems/shun-shi-zhen-da-yin-ju-zhen-lcof/solution/mian-shi-ti-29-shun-shi-zhen-da-yin-ju-zhen-she-di/

class Solution {
   public int[] spiralOrder(int[][] matrix) {
        if(matrix.length == 0) return new int[0];
        int l = 0, r = matrix[0].length - 1, t = 0, b = matrix.length - 1, x = 0;
        int[] res = new int[(r + 1) * (b + 1)];
        while(true) {
            //从左往右
            //列在变，列为循环值
            //从左往右的下一步是往下走，上边界内缩，故++t
            for(int i = l; i <= r; i++) res[x++] = matrix[t][i]; // left to right.
            if(++t > b){} break;

             //从上往下，行在变
            //从上往下的下一步是从右往左，右边界收缩，--r
            for(int i = t; i <= b; i++) res[x++] = matrix[i][r]; // top to bottom.
            if(l > --r) break;

            //从右向左，列在变
            //从右往左的下一步是从下往上，下边界收缩，--b
            for(int i = r; i >= l; i--) res[x++] = matrix[b][i]; // right to left.
            if(t > --b) break;

            //从下到上，行在变
            //从下到上的下一步是从左到右，左边界收缩，++l
            for(int i = b; i >= t; i--) res[x++] = matrix[i][l]; // bottom to top.
            if(++l > r) break;
        }
        return res;
    }

}