Problem:list of elements, one is unique, others always in pairs, find unique

 

	def unique(arr):
	""" find unique element in array
	1. only one is unique
	2. all the others are in pairs
	>>> unique([1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1])
	6
	A XOR A = 0
	0 XOR A = A
	"""
	x = 0
	for i in arr:
	x ^= i
	return x

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