Skip to main content

3. Single Number

easyAsked at Confluent

Find the one element appearing once when every other appears twice — Confluent uses it to probe whether you know XOR tricks before scaling to deduplicating Kafka records.

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

Problem

Given a non-empty array where every element appears twice except one, return the single one. Solve it with linear time and constant extra space.

Constraints

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

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 count

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

Time
O(n)
Space
O(n)
const c=new Map();
for (const x of nums) c.set(x,(c.get(x)||0)+1);
for (const [k,v] of c) if (v===1) return k;

Tradeoff:

2. XOR fold

XOR every element; pairs cancel and only the singleton remains. Constant space, single pass.

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

Tradeoff:

Confluent-specific tips

Confluent will probe the streaming variant — explain how XOR aggregates over a partition let you dedupe without holding the full key set in memory across a consumer-group rebalance.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

More Confluent coding interview questions

See all Confluent problems →

Companies that also ask Single Number