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18. Single Number

easyAsked at Plaid

Find the number that appears exactly once in an array where every other number appears twice. Plaid asks this because finding the un-matched ledger entry in a reconciliation pass has exactly this shape.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • Blind (2026)— Plaid intro — framed as reconciliation lookup.
  • LeetCode Discuss (2026)— Plaid OA warm-up.

Problem

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. You must implement a solution with a linear runtime complexity and use only constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -3 * 10^4 <= nums[i] <= 3 * 10^4
  • Each element appears twice except for one element which appears once.

Examples

Example 1

Input
nums = [2,2,1]
Output
1

Example 2

Input
nums = [4,1,2,1,2]
Output
4

Approaches

1. Hash map counting

Count occurrences in a hash map, return the one with count 1.

Time
O(n)
Space
O(n)
function singleNumber(nums) {
  const count = new Map();
  for (const n of nums) count.set(n, (count.get(n) || 0) + 1);
  for (const [n, c] of count) if (c === 1) return n;
}

Tradeoff: Violates the O(1) space constraint. Mention only as the obvious starting point.

2. XOR everything

XOR is commutative and self-cancelling: a ^ a == 0. XOR all numbers; duplicates cancel out, leaving the single number.

Time
O(n)
Space
O(1)
function singleNumber(nums) {
  let r = 0;
  for (const n of nums) r ^= n;
  return r;
}

Tradeoff: Bitwise trick that meets the constraint exactly. Demonstrates you reach for XOR for self-cancelling pair patterns.

Plaid-specific tips

Plaid uses this to verify bitwise-XOR fluency before harder reconciliation problems. Bonus signal: connect it to two-feed reconciliation — XOR the hashes of transactions from feed A with hashes from feed B, and the surviving bits identify the un-paired transactions.

Common mistakes

  • Initializing r to nums[0] then starting from i=1 — works but bigger surface area for off-by-ones than starting r=0.
  • Using addition instead of XOR — overflows and doesn't cancel.
  • Trying to sort first — that's O(n log n), missing the linear requirement.

Follow-up questions

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

  • Every element appears three times except one (LC 137) — harder, needs bit-by-bit counting mod 3.
  • Two elements appear once, rest twice (LC 260).
  • Reconcile two ledgers to find unpaired entries.

Solve it now

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Output

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FAQ

Why does XOR work?

XOR is associative and commutative, and a ^ a = 0. So XORing the whole array reduces to the single element XOR'd with 0, which is itself.

Does this work with negative numbers?

Yes — JS does bitwise XOR on 32-bit signed ints. Within the LC bound, that's safe.

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