## Problem – Single Number

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 * 10`^{4}

`-3 * 10`^{4} <= nums[i] <= 3 * 10^{4}

- Each element in the array appears twice except for one element which appears only once.

### Single Number LeetCode Solution in C++

```
class Solution {
public:
int singleNumber(vector<int>& nums) {
unordered_map<int,int> a;
for(auto x: nums)
a[x]++;
for(auto z:a)
if(z.second==1)
return z.first;
return -1;
}
};
```

### Single Number LeetCode Solution in Python

```
class Solution:
def singleNumber(self, nums: List[int]) -> int:
xor = 0
for num in nums:
xor ^= num
return xor
```

### Single Number LeetCode Solution in Java

```
public int singleNumber(int[] nums) {
int ans =0;
int len = nums.length;
for(int i=0;i!=len;i++)
ans ^= nums[i];
return ans;
}
```

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