Single Number

Description

Given a non-empty 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?

Example 1:

Input: [2,2,1]
Output: 1

Example 2:

Input: [4,1,2,1,2]
Output: 4

Solution(javascript)

/** TODO 空间不满足要求
 * @param {number[]} nums
 * @return {number}
 */
const singleNumber = function (nums = []) {
  const map = nums.reduce((acc, num) => {
    acc[num] = (acc[num] || 0) + 1
    return acc
  }, {})
  for (const [key, count] of Object.entries(map)) {
    if (count === 1) {
      return Number(key)
    }
  }
  return -1
}