leetcode: 16. 3Sum Closest

# Given an array S of n integers, 
# find three integers in S such that the sum is closest to a given number, target.
# Return the sum of the three integers. 
# You may assume that each input would have exactly one solution.
#
# For example, given array S = {-1 2 1 -4}, and target = 1.
#
# The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

class Solution():
    def threeSumClosest(self, x, target):
        x.sort()
        closest = sum(x[:3])
        for left in range(len(x) - 2):
            mid, right = left + 1, len(x) - 1
            while mid < right:
                s = x[left] + x[mid] + x[right]
                if s == target:
                    return s
                elif s < target:
                    mid += 1
                else:
                    right -= 1
                closest = s if abs(s-target) < abs(closest-target) else closest
        return closest


if __name__ == '__main__':
    assert Solution().threeSumClosest([-1, 2, 1, -4], 1) == 2