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10. Single Number

easyAsked at Grab

Find the lone element in an array where every other element appears twice — Grab uses this to test bitwise fluency.

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 the lone element. Solve in linear time and constant 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 then return the key with count 1.

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

Tradeoff:

2. XOR fold

XOR cancels duplicates; folding XOR across the array leaves the unique element.

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

Tradeoff:

Grab-specific tips

Grab interviewers reward candidates who reach the XOR insight without hints — frame the multi-service wallet as deduplicating transactions.

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Output

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