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.

// 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]
// ]

public class 3Sum {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>>  result = new ArrayList<>();

        Arrays.sort(nums);

        for(int i = 0; i < nums.length - 2; i++) {
            if(i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }

            int j = i + 1;
            int k = nums.length - 1;
            int target = -nums[i];

            while(j < k) {
                if(nums[j] + nums[k] == target) {
                    ArrayList<Integer> temp = new ArrayList<Integer>();

                    temp.add(nums[i]);
                    temp.add(nums[j]);
                    temp.add(nums[k]);

                    result.add(temp);

                    j++;
                    k--;

                    while(j < k && nums[j] == nums[j - 1]) {
                        j++;
                    }

                    while(j < k && nums[k] == nums[k + 1]) {
                        k--;
                    }
                } else if(nums[j] + nums[k] > target) {
                    k--;
                } else {
                    j++;
                }
            }
        }

        return result;
    }
}