Single Number - Implementation

Here's the complete solution:

function singleNumber(nums)
    result := 0
    for each num in nums
        result := result XOR num
    return result

This runs in $O(n)$ time and $O(1)$ space. The XOR operator (boolean logic at the bit level) cancels duplicate pairs, isolating the unique element. XOR turns a hard problem into three lines of code.