Stage 1 · Code
Arrays & Hashing
Matrix Traversal
Spiral traversal, in-place rotation, and zero-setting — the O(1)-space matrix techniques that appear in every FAANG loop.
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The Pattern
Matrix problems require careful boundary tracking. Spiral traversal uses four shrinking boundaries; in-place rotation uses a transpose + reflect trick; zeroing uses first row/col as markers to avoid extra space.
Spiral Traversal Path in a 4×4 Matrix
0
1
2
3
0
1Sstart
2
3
4
1
12
13
14
5
2
11
16
15Eend
6
3
10
9
8
7
Boundary-shrinking pattern
For spiral traversal, maintain top, bottom, left, right pointers. After traversing each edge, shrink the corresponding boundary inward. The loop continues while top ≤ bottom and left ≤ right.
Problems
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