Single Number

Given an array of integers A, 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?

• 1 <= |A| <= 2000000

• 0 <= A[i] <= INTMAX

First and only argument of input contains an integer array A.

Return a single integer denoting the single element.

Input 1:

 A = [1, 2, 2, 3, 1]

Input 2:

 A = [1, 2, 2]

Output 1:

 3

Output 2:

 1

Explanation 1:

3 occurs once.

Explanation 2:

1 occurs once.