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# 136. Single Number

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
```

#### 解题要点：

题目要求不能用extra memory，所以取0到list的倒数第二位，把i+1的元素作为储存xor结果的容器。返回list最后一位即可。

```python
class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        for i in range(len(nums)-1):
            nums[i+1] ^= nums[i]
        return nums[-1]
```