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Rotate Image

Certification: Intermediate Level Accuracy: 0% Submissions: 0 Points: 10

Write a C program to rotate an n x n 2D matrix representing an image by 90 degrees clockwise in-place. 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]]
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  • Output: [[7,4,1],[8,5,2],[9,6,3]]
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  • Explanation: Original matrix: 1 2 3 / 4 5 6 / 7 8 9. After 90° clockwise rotation: 7 4 1 / 8 5 2 / 9 6 3.
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Example 2
    
      
  • Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
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  • Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]
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  • Explanation: Original 4x4 matrix with elements arranged in rows. After rotation, first column becomes first row in reverse order. The transformation follows the pattern: matrix[i][j] -> matrix[j][n-1-i].
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Constraints
    
      
  • 1 ≤ n ≤ 20
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  • -1000 ≤ matrix[i][j] ≤ 1000
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  • The matrix is a square matrix (n x n)
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  • Time Complexity: O(n^2)
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  • Space Complexity: O(1)
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Hint
Solution

Solution Hints

    
      
  • First transpose the matrix (swap elements across main diagonal)
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  • Then reverse each row to get 90-degree clockwise rotation
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  • Alternative approach: rotate elements in concentric squares
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  • For transpose: swap matrix[i][j] with matrix[j][i]
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  • For row reversal: swap matrix[i][j] with matrix[i][n-1-j]
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  • Process only upper triangle during transpose to avoid double swapping
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Steps to solve by this approach:

 Step 1: Transpose the matrix by swapping elements across the main diagonal
 Step 2: Iterate through upper triangle to avoid double swapping during transpose
 Step 3: Swap matrix[i][j] with matrix[j][i] for all i < j
 Step 4: Reverse each row of the transposed matrix
 Step 5: Use two pointers (left and right) to reverse each row
 Step 6: Swap elements from both ends moving towards center
 Step 7: The combination of transpose and row reversal gives 90° clockwise rotation

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