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18. Single Number

easyAsked at Box

Find the only element appearing once in an array where every other appears twice — Box uses XOR-style tricks when matching unpaired sync events to detect orphans.

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

Problem

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. Must be linear time and use constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -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 counter

Count occurrences; return the key 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 all elements; pairs cancel, leaving the unique one. O(1) space.

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

Tradeoff:

Box-specific tips

Box loves XOR-fold answers — they map it to fingerprinting unpaired sync events without allocating per-event counters.

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Output

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