rotate2.go

/* https://leetcode.com/problems/rotate-image/description/
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]
    ]
*/

package larray

func rotate2(matrix [][]int) {
	n := len(matrix)
	mid := n/2 + n%2
	for i := 0; i < n; i++ {
		for j := 0; j < mid; j++ {
			swapi, swapj := n-(n-j), n-i-1
			matrix[i][j], matrix[swapi][swapj] = matrix[swapi][swapj], matrix[i][j]
		}
	}
	for i := n - 1; i >= mid; i-- {
		for j := n - 1; j >= mid; j-- {
			swapi, swapj := j, n-i-1
			matrix[i][j], matrix[swapi][swapj] = matrix[swapi][swapj], matrix[i][j]
		}
	}
}