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18. Single Number

easyAsked at Slack

Find the single number in an array where every other number appears twice.

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

Problem

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. Use linear runtime and 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, 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 single value remains. O(1) space.

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

Tradeoff:

Slack-specific tips

Slack interviewers love XOR for the constant-space property — useful in their mobile clients where memory budgets are tight.

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Output

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