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]
]

雙指標！

public class Solution {
   public static List<List<Integer>> threeSum(int[] nums) {
       Arrays.sort(nums);
       List<List<Integer>> result=new ArrayList<List<Integer>>();
       for(int i=0;i<nums.length-2&&nums[i]<=0;i++){
           if
 
(i>0 && nums[i]==nums[i-1])continue;
           int l=i+1;
           int r=nums.length-1;
           while(l<r){
               int sum=nums[l]+nums[i]+nums[r];
               if(sum==0){
                   List<Integer> list=new ArrayList<Integer>();
                   list
 
.add(nums[i]);
                   list.add(nums[l]);
                   list.add(nums[r]);
                   result.add(list);
                   l++;
                   r--;
                   while(l<r&&nums[l]==nums[l-1]) l++;
                   while(l<r&&nums[r]==nums[r+1]) r--;
               }else if(sum>0){
                   r--;
               }else{
                   l++;
               }
           }
       }
       return result;
   }
}