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8. Single Number

easyAsked at Baidu

Find the one element in an array where every other element appears exactly 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 element. Solve it with linear runtime and constant extra space.

Constraints

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

Count occurrences in a map, then scan for the entry with count 1.

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

Tradeoff:

2. XOR fold

XOR every value; paired values 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:

Baidu-specific tips

Baidu likes this for query-rewriting dedupe pipelines and they will explicitly grade you on stating the XOR identity (x ^ x = 0, x ^ 0 = x) before writing the loop.

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Output

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