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11. Single Number

easyAsked at Nubank

Find the element that appears once when every other element appears twice; Nubank uses it to probe bit-tricks intuition before deeper credit-ledger reconciliation problems.

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

Problem

Given a non-empty array of integers nums in which every element appears twice except for one, return that single element. Solve it in linear time and constant extra space.

Constraints

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

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

Tally counts in a Map, return the one with count 1.

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

Tradeoff:

2. XOR fold

XOR is associative, commutative, and self-inverse, so XOR-ing everything cancels the duplicates and leaves the loner. O(1) extra space.

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

Tradeoff:

Nubank-specific tips

Nubank likes the XOR answer — it shows you can reach for bitwise identities, useful when reconciling double-posted transaction events on PIX rails.

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Output

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