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55. Set Matrix Zeroes

mediumAsked at Vercel

If an element in an m x n matrix is 0, set its entire row and column to 0. Vercel asks this for the in-place O(1) extra space trick — using the first row and column as marker storage.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Source citations

Public interview reports confirming this problem appears in Vercel loops.

  • Glassdoor (2025-Q4)— Vercel platform onsite; O(1) space is the bonus.
  • Blind (2026-Q1)— Listed in Vercel platform engineer screen.

Problem

Given an m x n integer matrix, if an element is 0, set its entire row and column to 0's. You must do it in place.

Constraints

  • m == matrix.length
  • n == matrix[0].length
  • 1 <= m, n <= 200
  • -2^31 <= matrix[i][j] <= 2^31 - 1

Examples

Example 1

Input
matrix = [[1,1,1],[1,0,1],[1,1,1]]
Output
[[1,0,1],[0,0,0],[1,0,1]]

Example 2

Input
matrix = [[0,1,2,0],[3,4,5,2],[1,3,1,5]]
Output
[[0,0,0,0],[0,4,5,0],[0,3,1,0]]

Approaches

1. Two extra arrays for row/col flags

Mark rows and columns to zero in O(m+n) extra arrays; second pass zeros them.

Time
O(m*n)
Space
O(m+n)
function setZeroes(matrix) {
  const m = matrix.length, n = matrix[0].length;
  const rows = new Set(), cols = new Set();
  for (let i = 0; i < m; i++) for (let j = 0; j < n; j++) if (matrix[i][j] === 0) { rows.add(i); cols.add(j); }
  for (let i = 0; i < m; i++) for (let j = 0; j < n; j++) if (rows.has(i) || cols.has(j)) matrix[i][j] = 0;
}

Tradeoff: O(m+n) space. Vercel asks for O(1) follow-up.

2. Use first row/column as markers (optimal)

Use row 0 and col 0 to flag which rows/cols should be zeroed. Track separately whether row 0 / col 0 themselves should be zeroed. Two passes.

Time
O(m*n)
Space
O(1)
function setZeroes(matrix) {
  const m = matrix.length, n = matrix[0].length;
  let firstRowZero = false, firstColZero = false;
  for (let j = 0; j < n; j++) if (matrix[0][j] === 0) firstRowZero = true;
  for (let i = 0; i < m; i++) if (matrix[i][0] === 0) firstColZero = true;
  for (let i = 1; i < m; i++) {
    for (let j = 1; j < n; j++) {
      if (matrix[i][j] === 0) { matrix[i][0] = 0; matrix[0][j] = 0; }
    }
  }
  for (let i = 1; i < m; i++) {
    for (let j = 1; j < n; j++) {
      if (matrix[i][0] === 0 || matrix[0][j] === 0) matrix[i][j] = 0;
    }
  }
  if (firstRowZero) for (let j = 0; j < n; j++) matrix[0][j] = 0;
  if (firstColZero) for (let i = 0; i < m; i++) matrix[i][0] = 0;
}

Tradeoff: O(1) extra space. Two boolean variables to remember whether row 0 / col 0 had a zero originally — these get overwritten by the marker storage, so they must be saved first.

Vercel-specific tips

Vercel grades the O(1) version. Bonus signal: explaining WHY you need the two separate booleans (row 0 / col 0 are the marker storage, so their original zero state gets clobbered) and the order of the four passes.

Common mistakes

  • Forgetting the firstRowZero / firstColZero capture — overwrites the markers with the actual zeros.
  • Doing the second pass without skipping row/col 0 — zeros propagate incorrectly.
  • Mixing up the marker dimensions — row 0 marks columns, col 0 marks rows.

Follow-up questions

An interviewer at Vercel may pivot to one of these next:

  • Game of Life (LC 289) — similar in-place flag pattern.
  • What if 0 itself is a valid stored value? Use a sentinel.
  • Set Matrix Ones — zero out everything except cells in a row/col with a 1.

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Output

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FAQ

Why two extra booleans?

Row 0 and col 0 serve as the marker storage. If we just looked at matrix[0][j] at the end, we wouldn't know if that 0 was original (zero the whole row 0) or a marker (zero only column j).

Could I use Number.MIN_SAFE_INTEGER as a sentinel?

Yes, if the problem guarantees that value never appears. But the first-row/col trick uses no sentinel and works universally.

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