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Approach: XOR-fold the array; duplicates cancel, leaving the single number.

Complexity: O(n) time, O(1) space

js

/**
 * @param {number[]} nums
 * @return {number}
 */
var singleNumber = function(nums) {
	return nums.reduce(function(ret, num) {
		return ret ^ num;
	});
};

py

from functools import reduce
import operator


class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        return reduce(operator.xor, nums)