[LeetCode]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)

以一个数为target然后循环求得是否有两个数等于之，while循环里是关键算法。

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);       for(int i=0;i<num.length-2;i++){       int target = -num[i];       int left = i+1;       int right = num.length - 1;       while(left<right){       if(target == num[left]+num[right]){       List<Integer> lists= new ArrayList<Integer>();       lists.add(num[i]);       lists.add(num[left]);       lists.add(num[right]);       if(res.isEmpty()||!res.contains(lists)){      res.add(lists);       }       left++;       right--;       }else if(target > num[left]+num[right]){       left++;       }else{       right--;       }       }       }        return res;    }}