Single Number

Source: leetcode 136. Single Number

Q. Given an array of integers, every element appears twice except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

Answer 1: with using extra memory

/**
 * @param {number[]} nums
 * @return {number}
 */
export default function singleNumber (nums) {
  const seen = new Set()
  for (let i = 0; i < nums.length; i++) {
    if (seen.has(nums[i])) {
      seen.delete(nums[i])
    } else {
      seen.add(nums[i])
    }
  }
  return seen.values().next().value
}

Answer 2: without using extra memory

/**
 * @param {number[]} nums
 * @return {number}
 */
export default function singleNumber (nums) {
  return nums.reduce((curr, prev) => curr ^ prev)
}