15 3Sum

Given an arraynumsofn_integers, are there elements_a,b,c_innumssuch 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 {
    //[0,0,0,0,0,0,0,0]
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ans = new ArrayList<>();
        if (nums == null || nums.length < 3) {
            return ans;
        }
        Arrays.sort(nums);
        int len = nums.length;
        for (int i = 0; i < len; i++) {
            if (i >= 1) {
                if (nums[i] ==nums[i - 1]){
                    continue;
                }
            }
            if (nums[i] > 0) {
                break;
            }
            int left = i + 1;
            int right = nums.length - 1;
            int target = -nums[i];
            find2sum(nums, ans, left, right, target);
        }
        return ans;
    }
    private void find2sum(int[] nums, List<List<Integer>> ans, int left, int right, int target) {
        while (left < right) {

            if (nums[left] + nums[right] == target) {
                List<Integer> triple = new ArrayList<>();
                triple.add(-target);
                triple.add(nums[left]);
                triple.add(nums[right]);
                ans.add(triple);
                left++;
                right--;
                while(left < right && nums[left] == nums[left - 1]) {
                    left++;
                }
                while(left < right && nums[right] == nums[right + 1]) {
                    right--;
                }
            }else if (nums[left] + nums[right] > target){
                right--;  
            }else{
                left++;
            }
        }
    }
}