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18. Single Number

easyAsked at Lyft

Find the element that appears once when every other element 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. Solve it with linear runtime and constant space.

Constraints

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

Examples

Example 1

Input
nums = [2,2,1]
Output
1

Example 2

Input
nums = [4,1,2,1,2]
Output
4

Approaches

1. Hash map count

Count occurrences and return the one 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 all

XOR is its own inverse, so pairs cancel and only the singleton remains.

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

Tradeoff:

Lyft-specific tips

Lyft engineers cite XOR tricks in deduplicating event logs from rides; they'll ask you to prove why it works using XOR's associativity and self-inverse property.

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Output

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