^{From this stackoverflow question}

Given a 2D array of size $ M x N $, output the values in a anti-clock wise fashion. The output must start from the outside to the inside and the initial point always is going to be $(0,0)$.

Example Given:

$$ beginbmatrix colorblue1&colorred2&colorred3&colorred4 \ colorred5&6&7&colorred8 \ colorred9&10&11&colorred12 \ colorred13&colorred14&colorred15&colorred16endbmatrix $$

The edge values in counterclockwise is then $ 1,5,9,13,14,15,16,12,8,4,3,2 $.

Now we repeat the process for the inner values. This will end up with a matrix like the following

$$ beginbmatrix colorblue6&colorred7 \ colorred10&colorred11 endbmatrix$$

And the inner values is then $ 6,10,11,7 $

The final result will be then $ 1,5,9,13,14,15,16,12,8,4,3,2,6,10,11,7 $

Rules

• Assume non-empty input


• Assume matrix values as positive integers


• Standard I/O Methods apply


• Standard code-golf rules and winning criteria apply

Some test cases

Input
[
 [1, 2, 3, 4, 5, 6, 7],
 [8, 9, 10,11,12,13,14],
 [15,16,17,18,19,20,21]
]
Output
1,8,15,16,17,18,19,20,21,14,7,6,5,4,3,2,9,10,11,12,13

--------------------------------------------------------

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

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

Python 2, 52 bytes

f=lambda a:a and zip(*a)[0]+f(zip(*a[::-1])[1:])or()

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R, 59 bytes

f=function(m)"if"(ncol(m)-1,c(m[,1],f(t(m[nrow(m):1,-1]))))

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Recursively strip off the first column and rotate the rest of the matrix until you end up with only one column. Ungolfed:

f <- function(m) 
 if(ncol(m) == 1) 
 m
 else 
 c(m[,1], f(t(m[nrow(m):1,-1])))
 

Jelly, 9 bytes

ZḊUƊƬḢ€€Ẏ

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Pyth, 20 bytes

J.TQWJ=+YhJ=J_.TtJ)Y

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Explanation

J.TQWJ=+YhJ=J_.TtJ)Y
J.TQ Call the transposed input J.
 WJ ) While J is not empty...
 =+YhJ ... put the top row into Y (initially )...
 =J tJ ... remove the top row...
 _.T ... reverse and transpose (rotate clockwise).
 Y Output the result.