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14. Single Number

easyAsked at Activision

Find the element that appears once when every other element appears twice — Activision uses this to gauge bit-manipulation fluency before anti-cheat telemetry dedup questions.

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

Problem

Given a non-empty array where every element appears twice except one, find that single element. Your algorithm should run in linear time and use only constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Every element appears twice except 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 count

Count occurrences; return the key with count 1.

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

Tradeoff:

2. XOR accumulator

a XOR a = 0, a XOR 0 = a; folding XOR across the array leaves the lonely element. O(1) space.

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

Tradeoff:

Activision-specific tips

Activision favors candidates who reach for XOR here — the same identity-invariant trick reappears in anti-cheat telemetry dedup and packet-id reconciliation.

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Output

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