First thing that came to mind for me (just assuming there was needed a practical way to do it) was the following (translated into Python): # associative arra

First thing that came to mind for me (just assuming there was needed a practical way to do it) was the following (translated into Python): # associative array / dict / hash store = {} for elem in inputlist: if elem in store: del store[elem] else: store[elem] = True # return the only key left return store.keys()[0] Now that I've looked at the post, the XOR approach is quite efficient and a more fun answer.