[LeetCode] 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]
]

Thought process:
Iterate the matrix from the outer-most layer to the inner most layer. Rotate the layers one by one.

Solution (C++):

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class Solution { public: void rotate(vector<vector<int>>& matrix) { reverse(matrix.begin(), matrix.end()); for (int i = 0; i < matrix.size(); i++) { for (int j = i; j < matrix.size(); j++) { swap(matrix[i][j], matrix[j][i]); } } } };

Solution (Java):

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class Solution { public void rotate(int[][] matrix) { for (int i = 0; i < matrix.length / 2; i++) { for (int j = i; j < matrix.length - i - 1; j++) { rotate(matrix, i, j); } } } private void rotate(int[][] matrix, int i, int j) { int n = matrix.length; int temp = matrix[i][j]; matrix[i][j] = matrix[n - 1 - j][i]; matrix[n - 1 - j][i] = matrix[n - 1 - i][n - 1 - j]; matrix[n - 1 - i][n - 1 - j] = matrix[j][n - 1 - i]; matrix[j][n - 1 - i] = temp; } }

Time complexity: O(n^2), where n is the length of the matrix.