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# Single Number

Problem Description

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?

Problem Constraints

• 1 <= |A| <= 2000000

• 0 <= A[i] <= INTMAX

Input Format

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

Output Format

Return a single integer denoting the single element.

Example Input

Input 1:

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

Input 2:

` A = [1, 2, 2]`

Example Output

Output 1:

` 3`

Output 2:

` 1`

Example Explanation

Explanation 1:

```3 occurs once.
```

Explanation 2:

```1 occurs once.
```

NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.
Hints
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## Discussion

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