3Sum.java

/*
Given an array nums of n integers, are there elements a, b, c in nums 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.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]
*/

class Solution {
    public List<List<Integer>> threeSum(int[] nums) 
    {
        List<List<Integer>> res = new ArrayList<>();
        
        Arrays.sort(nums);
        
        for (int i = 0; i < nums.length - 2; i++)
        {
            if (i == 0 || nums[i] != nums[i - 1])
            {
                int lo = i + 1;
                int hi = nums.length - 1;
                int sum = 0 - nums[i];
                
                while (lo < hi)
                {
                    if (nums[lo] + nums[hi] == sum)
                    {
                        res.add(Arrays.asList(nums[i], nums[lo], nums[hi]));

                        while (lo < hi && nums[lo] == nums[lo + 1])
                        {
                            lo++;
                        }

                        while (lo < hi && nums[hi] == nums[hi - 1])
                        {
                            hi--;
                        }

                        lo++;
                        hi--;
                    }
                    else if (nums[lo] + nums[hi] < sum)
                    {
                        lo++;
                    }
                    else
                    {
                        hi--;
                    }
                }
            }
        }
        
        return res;
    }
}