Given an array S of n integers, 
are there elements a, b, c in S such that a + b + c = 0? 
Note:
Elements in a triplet (a,b,c) 
must be in non-descending order. (ie, a ≤ b ≤ c)
The solution set must not contain duplicate triplets.
For example, 
given array S = {-1 0 1 2 -1 -4},
    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)
方案：