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。

示例 1：

输入：matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出：[1,2,3,6,9,8,7,4,5]

示例 2：

输入：matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
输出：[1,2,3,4,8,12,11,10,9,5,6,7]

限制：

0 <= matrix.length <= 100
0 <= matrix[i].length <= 100

题解：

public class Solution {
    public int[] spiralOrder(int[][] matrix) {
        int row = matrix.length;
        if (row == 0) {
            return new int[0];
        }
        int col = matrix[0].length;
        int[] res = new int[row * col];
        int idx = 0;
        int left = 0, top = 0, right = col - 1, bottom = row - 1;
        while (true) {
            //从左往右走
            for (int i = left; i <= right; i++) {
                res[idx++] = matrix[top][i];
            }
            if (++top > bottom) {
                break;
            }
            //从上往下走
            for (int i = top; i <= bottom; i++) {
                res[idx++] = matrix[i][right];
            }
            if (--right < left) {
                break;
            }
            //从右往左走
            for (int i = right; i >= left; i--) {
                res[idx++] = matrix[bottom][i];
            }
            if (--bottom < top) {
                break;
            }
            //从下往上走
            for (int i = bottom; i >= top; i--) {
                res[idx++] = matrix[i][left];
            }
            if (++left > right) {
                break;
            }
        }
        return res;
    }
}