3Sum
【题目】
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)
【分析】
1.使用两头逼近的方法,利用两层循环。第一层循环遍历每一个元素,在第二层循环里两头逼近找出满足条件的结果
2.要考虑数据元素重复的问题,若第二层循环中遇到相同元素,需要继续向前;
3.第一层循环中晕倒相同元素,也需要继续向前
public class Solution { public List<List<Integer>> threeSum(int[] num) { List<List<Integer>> res=new ArrayList<List<Integer>>(); Arrays.sort(res); for(int i=0;i<num.length;i++) { int start=i+1; int end=num.length-1; int target=0-num[i]; while(start<end) { if(target==num[start]+num[end]) { Lsit<Integer> temp=new ArrayList<Integer>(); temp.add(num[i]); temp.add(num[start]); temp.add(num[end]); res.add(temp); start++; end--; while(start<end&&num[start]==num[start-1]) start++; while(start<end&&num[end]==num[end+1]) end--; }else if(num[start]+num[end]>target) end--; else start++; } while(i<num.length-1&&num[i]==num[i++]) i++; } return res; } }
【算法实现】