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.

Notice

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.

Example

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

(-1, 0, 1)

(-1, -1, 2)

Notes:

1. Fixed one point and then use two pointers
2. Don't forget to sort
3. Unique: ignore the duplicate numbers for each point

    public List<List<Integer>> threeSum(int[] nums) {
        if (nums == null || nums.length < 3) {
            return Collections.emptyList();
        }

        Arrays.sort(nums);
        List<List<Integer>> result = new ArrayList<>();
        for (int i = 0; i < nums.length-2; ++i) {
            if (i == 0 || i > 0 && nums[i] != nums[i-1]) {
                int j = i+1;
                int k = nums.length-1;
                while(j < k) {
                    int sum = nums[i] + nums[j] + nums[k];
                    if (sum == 0) {
                        result.add(Arrays.asList(nums[i], nums[j], nums[k]));
                        while(j < k && nums[j] == nums[j+1]) {
                            j++;
                        }

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

                        j++;
                        k--;
                    } else if (sum < 0) {
                        j++;
                    } else {
                        k--;
                    }
                }
            }
        }

        return result;
    }