alt Hacker News> given a list where every value appears exactly twice except one, XOR all the values together and the duplicates cancel out, leaving the unique element
For some reason this reminds me of the Fourier transform. I wonder if it can be performed with XOR tricks and no complicated arithmetic?
There's a more general formulation, which is that every value but one must appear even numbers of times, and the one must appear some odd number of times.