3Sum.java

/*
    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.
	    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.
    
    Link: https://leetcode.com/problems/3sum/

    Example: For example, given array S = {-1 0 1 2 -1 -4}, A solution set is: (-1, 0, 1), (-1, -1, 2)

    Solution: None
    
    Source: None
*/

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> resultList = new ArrayList<List<Integer>>();
        if (nums == null || nums.length < 3) {
            return resultList;
        }
        Arrays.sort(nums);
        for (int i = 0; i < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            int j = i + 1;
            int k = nums.length - 1;
            while (j < k) {
                int ans = nums[i] + nums[j] + nums[k];
                if (ans == 0) {
                    ArrayList<Integer> temp = new ArrayList<Integer>();
                    temp.add(nums[i]);
                    temp.add(nums[j]);
                    temp.add(nums[k]);
                    resultList.add(temp);
                    j++;
                    while (j < nums.length && nums[j] == nums[j - 1]) {
                        j++;
                    }
                    k--;
                    while (k >= 0 && nums[k] == nums[k + 1]) {
                        k--;
                    }
                } else if (ans > 0) {
                    k--;
                } else {
                    j++;
                }
            }
        }
        return resultList;
    }
}