3Sum——leetcode

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)

public class Solution {
    public List<List<Integer>> threeSum(int[] num) {
        Set<List<Integer>> set=new HashSet<List<Integer>>();
        Arrays.sort(num);
        List<List<Integer>> ans=new ArrayList<List<Integer>>();
        if(num.length<=2)
        {
            return ans;
        }
        for(int i=0;i<num.length-2;i++)
        {
            if(i>0&&num[i]==num[i-1])
            {
                continue;
            }
            int j=i+1;
            int k=num.length-1;//j,k双指针，巧妙降低复杂度至N^2
            while(j<k)
            {
                int sum=num[i]+num[j]+num[k];
                if(sum==0)
                {
                    List<Integer> list=new ArrayList<Integer>();
                    list.add(num[i]);
                    list.add(num[j]);
                    list.add(num[k]);
                    set.add(list);
                    j++;k--;
                }
                else if(sum<0)
                {
                    j++;
                }
                else
                {
                    k--;
                }
            }
        }
        ans.addAll(set);//这个函数不错，可以把set中的元素全都添加到List。List的函数addAll(<Collection>)
        return ans;
    }
}