Spiral Matrix
Description: Given a matrix, return all its elements in spiral order: left to right along the top row, top to bottom along the right column, and so on.
Example 1:
Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [1,2,3,6,9,8,7,4,5]
Example 2:
Input: matrix = [[1,2],[3,4]]
Output: [1,2,4,3]
Constraints:
1 <= число строк, столбцов <= 10
-100 <= matrix[i][j] <= 100
Recommended time and space complexity
O(m × n) time and O(1) extra space.
Hint 1
Keep four boundaries: top, bottom, left, right.
Hint 2
After traversing a side, move the corresponding boundary inward.
Hint 3
Check the boundaries before the bottom row and left column, otherwise elements get visited twice.