FAANGPrep Sprint
Rotate Image
Rotate the matrix by 90 degrees clockwise in-place.
MediumSolve on LeetCode
Problem Understanding
Turn an n x n matrix, an image stored as a grid of pixels, a quarter turn clockwise. The rotation must happen in-place, inside the same matrix, without allocating a second one.
Strategy (Transpose + Reflect)
- Transpose: Swap
matrix[i][j]withmatrix[j][i]. - Reflect: Reverse each row.
Step 1: Transpose converts rows to columns.
Step 2: Reversing rows fixes the left-right order for 90 deg rotation.
Interactive Visualization
READY
0
1
2
0
1
2
3
1
4
5
6
2
7
8
9
Size
3 × 3
Swaps
0
n = 3. First transpose: swap each cell above the diagonal with its mirror below it (the diagonal swaps with itself, so it never moves)
Next
Swap matrix[0][1] and matrix[1][0]: 2 ↔ 4
Matrix
See the two-step process: Transpose then Reverse.