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5. Single Number

easyAsked at Revolut

Find the one element that appears once when every other appears twice, a Revolut warm-up that mirrors spotting an unmatched payment leg in a duplicated batch.

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

Problem

Given a non-empty array of integers nums where every element appears twice except for one, find that single one. You must implement it with linear runtime and constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • Every 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 each value and return the one with count one.

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

Tradeoff:

2. XOR

XOR all elements; pairs cancel and the unique value remains. O(1) extra space.

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

Tradeoff:

Revolut-specific tips

Revolut likes you to connect XOR to reconciliation parity bits — call out that this trick only works because debits and credits are matched as equal integer pairs.

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Output

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