0048-rotate-image

Try it on leetcode

Description

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

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:

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

Example 2:

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

 

Constraints:

  • n == matrix.length == matrix[i].length
  • 1 <= n <= 20
  • -1000 <= matrix[i][j] <= 1000

Solution(Python)

class Solution:
    def rotate(self, matrix: List[List[int]]) -> None:
        """
        Do not return anything, modify matrix in-place instead.
        """
        # Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
        # Output: [[7,4,1],[8,5,2],[9,6,3]]
        # 
        #  dignal flip and roatate vertical
        #  1 2  3 1
        #  3 4  4 2
        # 
        #  1 3
        #  2 4
        # 
        #  3 1
        #  4 2
        # i,j
        # 00 01 
        # 10 11
        # i,j = j, i
        # 
        # 3*3
        # 
        #  00 
        #  0 1 02 
        #  01 02 03
        n = len(matrix)
        # dignal flip
        for i in range(n): # 0 1 2
            for j in range(i): # 
                if i != j:
                    matrix[i][j],  matrix[j][i] = matrix[j][i], matrix[i][j]

        # vertical flip
        # 
        for row in range(n):
            i,j = 0, n-1

            while i < j:
                 matrix[row][i],  matrix[row][j] = matrix[row][j], matrix[row][i]
                 i+=1
                 j-=1

Dry Run