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Problem

Spiral Matrix

LeetCode #54Medium
Four shrinking boundaries

Given an m x n matrix, return all of its elements in spiral order, traversing the outer ring clockwise and spiraling inward.

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step 1 / 21
3 × 4 grid
1
2
3
4
5
6
7
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9
10
11
12
spiral order
Boundary simulation
▸
1top=0, bottom=R−1, left=0, right=C−1
2while top ≤ bottom and left ≤ right:
3 
4    for c in left..right:  take (top, c)
5    top++
6    for r in top..bottom:  take (r, right)
7    right−−
8    if top ≤ bottom:
9        for c in right..left: take (bottom, c)
10        bottom−−
11    if left ≤ right:
12        for r in bottom..top: take (r, left)
13        left++
14return collected
state
  • rows3
  • cols4

line 1Read the grid in a SPIRAL: across the top, down the right, back along the bottom, up the left, then inward and repeat. The clean way is four boundaries — top, bottom, left, right — that close in after each edge is consumed.