LeetCode 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 LinkedList<>();
        int n = num.length;
        
        if (n < 3)
            return list;
            
        Arrays.sort(num);
        
        int left, right, sum;
            
        for (int i = 0; i <= n-3; i++)
        {
            left = i+1;
            right = n-1;
            
            if (i==0 || num[i]>num[i-1])
            {
                while (left < right)
                {
                    sum = num[i]+num[left]+num[right];
                    
                    if (sum < 0)
                        left++;
                    else if (sum > 0)
                        right--;
                    else
                    {
                        List<Integer> ret = new LinkedList<>();
                        ret.add(num[i]);
                        ret.add(num[left]);
                        ret.add(num[right]);
                        
                        list.add(ret);    
                        left++;
                        right--;
                        
    			// Avoid duplicate solutions
    			while (left<right && num[right]==num[right+1])
    			        right--;
     
    			while (left<right && num[left]==num[left-1])
    				left++;                        
                    }
                }
            }
        }
        
        return list;
    }
}