LeetCode 136 ·
Easy
Single Number
Every element appears twice except one; find it.
Try it
Step through the core mechanic. The simulator below runs the bit manipulation shape this problem is built on.
Walk the pattern
No dedicated step-through for this one yet. The shape is Bit manipulation — its pattern page has the interactive walkthrough, the reference implementation, and a five-problem progression that this problem sits inside.
The approach
XOR everything together. Pairs cancel (x ^ x = 0) and 0 ^ y = y, leaving the unique value. O(1) space, no hashing.
| Aspect | Value |
|---|---|
| Pattern | Bit manipulation |
| Recognise it by | One element appears once, all others twice. |
| Time complexity | O(n) |
| Space complexity | O(1) |
| Difficulty | Easy |
Who asks it
Companies known to ask this problem, from public LeetCode company-tag aggregations. A signal of where to expect it, not a guarantee.