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18. Single Number

easyAsked at Salesforce

Find the element that appears only once when every other element appears twice. Salesforce uses this to test the XOR trick and the recognition that O(1) space is achievable.

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

Source citations

Public interview reports confirming this problem appears in Salesforce loops.

  • Glassdoor (2026-Q1)— Salesforce uses this as a litmus test for bit-manipulation awareness.
  • Blind (2025-10)— Recurring on Salesforce backend phone screens.

Problem

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. You must implement a solution with a linear runtime complexity and use only constant extra space.

Constraints

  • 1 <= nums.length <= 3 * 10^4
  • -3 * 10^4 <= nums[i] <= 3 * 10^4
  • Each element in the array appears twice except for one element which appears once.

Examples

Example 1

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

Example 2

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

Example 3

Input
nums = [1]
Output
1

Approaches

1. Hash count

Build a frequency map; return the key with count 1.

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

Tradeoff: Linear time but violates the O(1) space requirement. Salesforce will explicitly fail this.

2. XOR fold

XOR all elements together. Pairs cancel (a XOR a = 0); the single element remains.

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

Tradeoff: Three lines. Exploits the property that XOR is commutative and self-inverse. Salesforce wants this in 30 seconds.

Salesforce-specific tips

Salesforce flags this as a 'do you know your bitwise tricks' filter. They specifically want you to articulate the two properties (commutative + self-inverse) that make XOR work. Bonus signal: mention that this generalizes to 'every element appears k times except one' for some k (using mod-k counters per bit).

Common mistakes

  • Using ^ but storing in a non-number (e.g., string) — silent type coercion.
  • Initializing result to a non-zero value — gives result XORed with that value.
  • Falling back to the hash map because XOR feels 'tricky' — Salesforce expects the optimal answer.

Follow-up questions

An interviewer at Salesforce may pivot to one of these next:

  • Single Number II (LC 137) — every other element appears three times.
  • Single Number III (LC 260) — two elements appear once.
  • Missing Number (LC 268) — XOR with 0..n.

Solve it now

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Output

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FAQ

Why does XOR work here?

Because XOR is commutative (order doesn't matter) and self-inverse (a XOR a = 0). Every pair cancels itself, leaving only the unpaired element.

Could I sort first?

Sorting + comparing adjacent pairs works but is O(n log n), violating the linear-time requirement.

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