Skip to main content

10. Single Number

easyAsked at Monzo

Find the one transaction ID that appears exactly once in a list where every other ID appears twice.

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 a solution with linear runtime complexity and use only constant extra space.

Constraints

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

Count occurrences with a hash map and return the key with count 1.

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 fold

XOR all elements; duplicates cancel and the lone value remains. Constant space and one linear pass.

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

Tradeoff:

Monzo-specific tips

Monzo values O(1)-space tricks that translate cleanly to ledger reconciliation passes over large transaction batches.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Single Number and other Monzo interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →