Leetcode: Rotate Image

Clockwise rotate a 2D matrix



Similar Problems:


You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1:

Given input matrix = 
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:

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

Example 2:

Given input matrix =
[
  [ 5, 1, 9,11],
  [ 2, 4, 8,10],
  [13, 3, 6, 7],
  [15,14,12,16]
], 

rotate the input matrix in-place such that it becomes:

[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.


## Blog link: https://code.dennyzhang.com/rotate-image
## Basic Ideas: Rotate circle by circle. From outer to inner layer.
##             Use an array of n to save data temporarily
## Complexity: Time O(n*n), Space O(n)
class Solution(object):
    def rotate(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: void Do not return anything, modify matrix in-place instead.
        """
        length = len(matrix[0])
        if length <=1:
            return
        start_i = 0
        start_j = 0
        tmp_list = [None] * (length*4-4)

        while (length > 1):
            total_count = length*4-4
            # rotate current layer
            k = 0
            # save top
            for j in range(start_j, start_j + length):
                tmp_list[k] = matrix[start_i][j]
                k = k + 1
            #print("tmp_list: %s" % tmp_list)

            # save right
            for i in range(start_i+1, start_i + length):
                tmp_list[k] = matrix[i][start_j+length-1]
                k = k + 1
            # save bottom
            for j in range(start_j+length-2, start_j-1, -1):
                tmp_list[k] = matrix[start_i+length-1][j]
                k = k + 1
            # save left
            for i in range(start_i + length-2, start_i, -1):
                tmp_list[k] = matrix[i][start_j]
                k = k + 1

            # rotate
            k = length*3-2-1

            # change top
            for j in range(start_j, start_j + length):
                matrix[start_i][j] = tmp_list[k]
                k = (k + 1) % total_count
            # change right
            for i in range(start_i+1, start_i + length):
                matrix[i][start_j+length-1] = tmp_list[k]
                k = (k + 1) % total_count
            # change bottom
            for j in range(start_j+length-2, start_j-1, -1):
                matrix[start_i+length-1][j] = tmp_list[k]
                k = (k + 1) % total_count
            # change left
            for i in range(start_i + length-2, start_i, -1):
                matrix[i][start_j] = tmp_list[k]
                k = (k + 1) % total_count

            # go to inner layer
            start_i += 1
            start_j += 1
            length -= 2

if __name__ == '__main__':
    s = Solution()

    print s.rotate([[ 5, 1, 9,11],
                     [ 2, 4, 8,10],
                     [13, 3, 6, 7],
                     [15,14,12,16]])
    print s.rotate([[1,2,3],
                    [4,5,6],
                    [7,8,9]])
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