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8. Single Number

easyAsked at Squarespace

Find the one element that appears once when every other element appears twice; Squarespace uses it to test whether you reach for XOR over a hash set.

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

Problem

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

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Exactly one element appears once; all others appear twice

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 frequencies then return the element 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. Bitwise XOR

XORing every element cancels duplicates and leaves the unique value, all in constant space.

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

Tradeoff:

Squarespace-specific tips

Squarespace likes when you mention this XOR trick also dedupes their content-block fingerprints during template-render diffing.

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Output

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