15. 3Sum

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) {
        int n = nums.length;
        Arrays.sort(nums);
        List<List<Integer>> ans = new LinkedList<>();
        for (int i = 0; i < n - 2; ++i) {
            if (i == 0 || nums[i] != nums[i - 1]) {
                int sum = 0 - nums[i];
                int l = i + 1, r = n - 1;
                while (l < r) {
                    if (nums[l] + nums[r] == sum) {
                        ans.add(Arrays.asList(nums[i], nums[l], nums[r]));
                        while (l + 1 <= r && nums[l] == nums[l + 1])l++;
                        while (r - 1 >= l && nums[r] == nums[r - 1])r--;
                        l++;
                        r--;
                    } else if (nums[l] + nums[r] < sum) {
                        l++;
                        
                    } else {
                        r--;
                    }
                }
            } 
            
        }
        return ans;
    }
}