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Problem

Rotate Image

LeetCode #48Medium
90° in place · transpose+reverse or 4-cycles

Given an n x n matrix representing an image, rotate it 90 degrees clockwise in place, modifying the matrix directly without allocating another 2D array.

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step 1 / 18
3 × 3 image
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Transpose + reverse rows
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1given matrix (n × n)
2// phase 1: transpose
3for r in 0..n−1: for c in r+1..n−1:
4    swap M[r][c], M[c][r]
5// phase 2: reverse each row
6for each row:
7    swap M[r][c], M[r][n−1−c]
8done
state
  • plantranspose → reverse rows

line 1Rotate the image 90° clockwise IN PLACE (no second matrix). A clean two-step recipe: first TRANSPOSE (mirror across the main diagonal), then REVERSE each row. Together that equals a clockwise quarter-turn.