三数之和为零问题-优快云博客

探讨了如何在整数数组中寻找所有唯一且不重复的三元组，使这三个数相加等于零。该解决方案首先对数组进行排序，接着采用两指针法遍历数组以减少时间复杂度。

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)

public class Solution {
    public List<List<Integer>> threeSum(int[] num) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        
        if(num==null || num.length<3) return res;
        Arrays.sort(num);// sort first
        int a,b,c;
        int target=0;
        // O(n^2)  
        for(int i=0;i<num.length-1;i++){
            a=i;b=a+1;c=num.length-1;
            while(b<c){
                if(num[a]+num[b]+num[c]==target){
					List<Integer> arr=new ArrayList<Integer>();
					arr.add(num[a]);
                    arr.add(num[b]);
                    arr.add(num[c]);
                    res.add(arr);
                    b++;
                    c--;
                }
                else if(num[a]+num[b]+num[c]>target){
                    c--;
                }
                else{
                    b++;
                }
            }
        }
        HashSet set=new HashSet(res);
        res.clear();
        res.addAll(set);
        return res;
    }
}

思路：先对数组排序，然后左右夹逼。