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16. Spiral Matrix

mediumAsked at Postman

Return all elements of an m x n matrix in spiral order.

By Sam K., Founder, InterviewChamp.AI · Last verified

Problem

Given an m x n matrix, return all elements of the matrix in spiral order, starting at the top-left and walking clockwise.

Constraints

  • 1 <= m, n <= 10
  • -100 <= matrix[i][j] <= 100

Examples

Example 1

Input
matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output
[1,2,3,6,9,8,7,4,5]

Example 2

Input
matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
Output
[1,2,3,4,8,12,11,10,9,5,6,7]

Approaches

1. Visited matrix

Track visited cells in a parallel boolean matrix and turn when blocked or out of bounds.

Time
O(mn)
Space
O(mn)
// step in current direction; if next cell visited or OOB, turn right

Tradeoff:

2. Layer shrinking

Maintain top, bottom, left, right boundaries; walk one edge, then shrink that boundary; stop when they cross.

Time
O(mn)
Space
O(1)
function spiral(m) {
  const out = [];
  let t = 0, b = m.length - 1, l = 0, r = m[0].length - 1;
  while (t <= b && l <= r) {
    for (let j = l; j <= r; j++) out.push(m[t][j]); t++;
    for (let i = t; i <= b; i++) out.push(m[i][r]); r--;
    if (t <= b) { for (let j = r; j >= l; j--) out.push(m[b][j]); b--; }
    if (l <= r) { for (let i = b; i >= t; i--) out.push(m[i][l]); l++; }
  }
  return out;
}

Tradeoff:

Postman-specific tips

Postman expects clean boundary tracking — they ship a UI grid editor for API examples, and bookkeeping errors map directly to off-by-one bugs in cell selection.

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Output

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