How do you rotate a two dimensional array?

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Inspired by Raymond Chen’s post, say you have a 4×4 two dimensional array, write a function that rotates it 90 degrees. Raymond links to a solution in pseudo code, but I’d like to see some real world stuff.

[1][2][3][4]
[5][6][7][8]
[9][0][1][2]
[3][4][5][6]

Becomes:

[3][9][5][1]
[4][0][6][2]
[5][1][7][3]
[6][2][8][4]

Update: Nick’s answer is the most straightforward, but is there a way to do it better than n^2? What if the matrix was 10000×10000?

64 Answers
64

O(n^2) time and O(1) space algorithm ( without any workarounds and hanky-panky stuff! )

Rotate by +90:

  1. Transpose
  2. Reverse each row

Rotate by -90:

Method 1 :

  1. Transpose
  2. Reverse each column

Method 2 :

  1. Reverse each row
  2. Transpose

Rotate by +180:

Method 1: Rotate by +90 twice

Method 2: Reverse each row and then reverse each column (Transpose)

Rotate by -180:

Method 1: Rotate by -90 twice

Method 2: Reverse each column and then reverse each row

Method 3: Rotate by +180 as they are same

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