15. 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.

Note:

• 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.

    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)
代码如下：（超时）

 public class Solution {
     public List<List<Integer>> threeSum(int[] nums) {
         List<List<Integer>> list1=new ArrayList<>();
         List<Integer> list=new ArrayList<>();
         Map<List<Integer>,List<Integer>> map=new HashMap<>();
         int a=0;

         Arrays.sort(nums);

         for(int i=0;i<=nums.length-3;i++)
         {
             if(nums[i]<=0)
             {
                 list.add(nums[i]);
                 a=nums[i]*(-1);
             }
             else break;
             for(int j=i+1;j<=nums.length-2;j++)
             {
                 if(nums[j]<=a)
                 {
                     list.add(nums[j]);
                     a=a-nums[j];
                     for(int k=j+1;k<=nums.length-1;k++)
                     {
                          if(nums[k]>a)
                              break;
                          else if(nums[k]==a)
                         {
                             list.add(nums[k]);
                             if(!map.containsKey(list))
                             {
                                 map.put(list, list);
                                 list1.add(list);
                             }
                             break;
                         }

                     }
                 }
                 else break;
                 list=new ArrayList<>();
                 list.add(nums[i]);
                 a=nums[i]*(-1);
             }
             list=new ArrayList<>();
         }

         return list1;
     }
 }