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84. Move Zeroes

easyAsked at Reddit

Move zeros to the end while keeping the order of non-zeros. Reddit uses this to test in-place two-pointer technique — the same compact-and-tombstone pattern used when removing soft-deleted comments from a tree without rebuilding it.

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

Source citations

Public interview reports confirming this problem appears in Reddit loops.

  • Glassdoor (2026-Q1)— Reddit phone-screen warm-up.

Problem

Given an integer array nums, move all 0's to the end of it while maintaining the relative order of the non-zero elements. Note that you must do this in-place without making a copy of the array.

Constraints

  • 1 <= nums.length <= 10^4
  • -2^31 <= nums[i] <= 2^31 - 1

Examples

Example 1

Input
nums = [0,1,0,3,12]
Output
[1,3,12,0,0]

Example 2

Input
nums = [0]
Output
[0]

Approaches

1. Compact + fill zeros

First pass: copy non-zeros forward. Second pass: fill remaining with 0.

Time
O(n)
Space
O(1)
function moveZeroes(nums) {
  let k = 0;
  for (const n of nums) if (n !== 0) nums[k++] = n;
  while (k < nums.length) nums[k++] = 0;
}

Tradeoff: Two passes. Most writes.

2. Two-pointer swap (optimal)

Slow points to next zero position; fast scans. On nonzero, swap with slow and advance slow.

Time
O(n)
Space
O(1)
function moveZeroes(nums) {
  let slow = 0;
  for (let fast = 0; fast < nums.length; fast++) {
    if (nums[fast] !== 0) {
      [nums[slow], nums[fast]] = [nums[fast], nums[slow]];
      slow++;
    }
  }
}

Tradeoff: Single pass with minimum writes when zeros are rare.

Reddit-specific tips

Reddit interviewers want the two-pointer swap. Bonus signal: connect to soft-deleted comments — same compact-with-tombstone shape.

Common mistakes

  • Swapping when nums[fast] === 0 (does nothing useful and wastes work).
  • Forgetting to advance slow only on swap.
  • Using extra space when in-place is required.

Follow-up questions

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

  • Remove element (LC 27).
  • Move all negatives to front.
  • Sort colors (LC 75).

Solve it now

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Output

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FAQ

When does this minimize writes?

When zeros are rare. Each non-zero hop costs one swap; the leading prefix has many same-position swaps (no-ops) but no useless writes.

Why not skip swap when slow == fast?

Tiny optimization. The swap is a no-op anyway.

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