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12. Single Number

easyAsked at MercadoLibre

Find the integer that appears exactly once when every other appears twice.

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 one. Your solution should run in linear time and use constant extra space.

Constraints

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

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 in a map and return the entry 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 fold

XOR all elements; duplicates cancel and the lone element remains.

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

Tradeoff:

MercadoLibre-specific tips

MercadoLibre fraud teams love the XOR trick because it mirrors signature reconciliation — paired chargeback events cancel and the unmatched dispute pops out the same way.

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Output

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