Given an array S of n integers, are there elements a, b, c in S

such that a + b + c = 0?

Find all unique triplets in the array which gives the sum of zero.

Note: 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]
]
```