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How do you rotate an N×N matrix by 90 degrees in place?

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01

Understand the problem

Explain in-place matrix rotation.

matrixin-place
02

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03

Study the solution

Transpose the matrix (swap m[i][j] with m[j][i]), then reverse each row. That combination gives a clockwise 90° rotation in place with O(1) extra space and O(n²) time. Reversing columns instead rotates counter-clockwise.

Solution ready — 2 min read

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04

Read the code

Transpose then reverse rows
Run Playground
def rotate(matrix):
    n = len(matrix)
    for i in range(n):                 # transpose (upper triangle)
        for j in range(i + 1, n):
            matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
    for row in matrix:                 # reverse each row
        row.reverse()
    return matrix


# --- demo ---
print(rotate([[1, 2, 3], [4, 5, 6], [7, 8, 9]]))
# [[7, 4, 1], [8, 5, 2], [9, 6, 3]]
05

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