This is an exercise on LeetCode. I get an except of 

> 
> UnboundLocalError on line 15.
> 
> 
> 

Why? And how to fix it?

```

class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        self.nums = []
        self.target = int

        for i in range(len(nums)):
            for j in range(i + 1, len(nums)):
                if nums[i] + nums[j] == target:
                    a = []
                return a[i, j]

```