15. 3Sum

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:
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)

public class Solution {    public List<List<Integer>> threeSum(int[] num) {        List<List<Integer>> list = new ArrayList<>();        if (num.length<3)            return list;        Arrays.sort(num);        for (int i=0; i<num.length-2; i++) {            if(i!=0 && num[i]==num[i-1])                continue;            int left = i+1;            int right = num.length-1;            while (left<right) {                if (left>i+1 && num[left]==num[left-1]) {                    left++;                    continue;                }                if (right<num.length-2 && num[right]==num[right+1]) {                    right--;                    continue;                }                int sum = num[i] + num[left] + num[right];                if (sum==0) {                    List<Integer> item = new ArrayList<>();                    item.add(num[i]);                    item.add(num[left]);                    item.add(num[right]);                    list.add(item);                    left++;                    right--;                } else if (sum>0) {                    right--;                } else {                    left++;                }            } // while        } // for        return list;    }}