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18. Single Number

easyAsked at Ola

Find the lone element in an array where every other element 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. Your algorithm should have linear runtime complexity and use constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -3*10^4 <= nums[i] <= 3*10^4

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 in a map 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 fold

XOR all values; duplicates cancel and the lone value remains.

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

Tradeoff:

Ola-specific tips

Ola favors the XOR insight; mention how it removes the need for a dedup table when scanning paired pickup/drop-off event IDs.

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Output

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