Leetcode 48: Rotate Image

1. Problem Statement (Simple Explanation):

You’re given an n x n 2D matrix representing an image.

You must rotate the image by 90° clockwise in-place, meaning:

Modify the input matrix directly.
Do not allocate another n x n matrix.

2. Examples:

Example 1:

Input:

matrix = [

[1,2,3],

[4,5,6],

[7,8,9]

]

Output (90° clockwise):

[

[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]

]

3. Approach 1 – Transpose + Reverse Each Row (Clean & Standard):

A 90° clockwise rotation can be done in two in-place steps:

Transpose the matrix (swap rows and columns):

matrix[i][j] ↔ matrix[j][i] for i < j.
After transpose, rows become columns.

Reverse each row:

For each row, reverse the order of elements.

This combination is equivalent to rotating the matrix 90° clockwise.

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

Transpose:

[

[1,4,7],

[2,5,8],

[3,6,9]

]

Reverse each row:

[

[7,4,1],

[8,5,2],

[9,6,3]

]

Pseudo-code:

Complexity:

Time: O(n2) – every element is touched a constant number of times.
Space: O(1) extra – in-place.

4. Java code:

5. C code:

6. C++ code:

7. Python code:

8. JavaScript code:

Comments

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