Store possible sum of two value in set (two-some)

Init set with nums[0] + nums[1]

Iterate rest elements of given array, nums

Loop invariant: let nums[i] as n

- iterate on two-some and find nearest three-some
- add more two-some into set

```
class Solution(object):
def threeSumClosest(self, nums, target):
l = len(nums)
possible_sum_of_two_value = set() # store currently possible sum of two elements
nearest_sum_of_three_value = None
current_minimum_distance_to_target = float('inf')
possible_sum_of_two_value.add(nums[0] + nums[1])
for i in range(2, l):
n = nums[i]
for e in possible_sum_of_two_value:
if e + n == target:
return target
distance = abs(e + n - target)
if distance < current_minimum_distance_to_target:
current_minimum_distance_to_target = distance
nearest_sum_of_three_value = e + n
for p in nums[:i]:
possible_sum_of_two_value.add(n + p)
return nearest_sum_of_three_value
```