48.md

48. Rotate Image

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]
]

my thoughts:

1. iterate the left-up quarter of the matrix; rotate. Tricky part is for even n, the quarter is just the quarter; for odd n, the quarter need to be a l = w+1 retangle to avoid rotating mid line twice.

my solution:

**********66 ms
class Solution:
    def rotate(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: void Do not return anything, modify matrix in-place instead.
        """
        n = len(matrix)
        for i in range((n+1)//2):
            for j in range((n)//2):
                cor_set = [[j, n-i-1], [n-i-1, n-j-1], [n-j-1, i], [i, j]]
                pval = matrix[i][j]
                #print(cor_set)
                for k in range(4):
                    cor = cor_set[k]
                    temp = matrix[cor[0]][cor[1]]
                    matrix[cor[0]][cor[1]] = pval
                    pval = temp
                    

my comments:

from other ppl's solution:

1. use extra space speed up the solution a little, 52ms:
class Solution:
    def rotate(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: void Do not return anything, modify matrix in-place instead.
        """
        row = len(matrix)
        col = len(matrix[0])
        result =[]
        for c in range(col):
            r = row-1
            for i in range(row):
                result.append(matrix[r][c])
                r -= 1
            matrix.append(result)
            result = []
        for each in range(row):
            matrix.remove(matrix[0])
			
2. need to look into zip(), 49ms:
class Solution:
    def rotate(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: void Do not return anything, modify matrix in-place instead.
        """
        matrix[:] = zip(*matrix[::-1])