Time spent : 20 min()

GIVEN: A matrix of m x n elements.

RETURNS: All elements of the matrix in spiral order.

EXAMPLES:

Example 1:

Input:
[
 [ 1, 2, 3 ],
 [ 4, 5, 6 ],
 [ 7, 8, 9 ]
]
Output: [1,2,3,6,9,8,7,4,5]

Example 2:

Input:
[
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9,10,11,12]
]
Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Algorithm:

Using a list and boolean array. If the new coordinate is valid and not visited, change x and y. Otherwise change direction.