Single Number

Easy
Concept
Code
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Solution Guide: Single Number

Approach

XOR all elements together in a single pass. Since XOR is self-inverse (a ^ a = 0) and commutative, every duplicate pair cancels out, leaving only the unique element.

Complexity Analysis

Time
O(n)
Space
O(1)

Pattern

XOR Pattern

Why It Works

XOR of a number with itself is zero, and XOR with zero is the number itself, so all paired duplicates vanish and the lone element remains.

Updated Feb 2026