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Single Number

Problem​

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one.

You must implement a solution with a linear runtime complexity and use only constant extra space.

 

Example 1:

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

Example 2:

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

Example 3:

Input: nums = [1]
Output: 1

 

Constraints:

  • 1 <= nums.length <= 3 * 104
  • -3 * 104 <= nums[i] <= 3 * 104
  • Each element in the array appears twice except for one element which appears only once.

Solution​

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

We will use the two following properties of XOR:

  1. A XOR A = 0.
  2. A XOR 0 = A.

Thus, since nums contains a single value that is unique, while all other values has exactly two copies; XORing all the values together cancels out the duplicate values and so only the unique value is left.