ThreeSum

Problem

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

Solution

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
        for(int i = 0; i < nums.length - 2; i++){
            // 跳过重复元素
            if(i > 0 && nums[i] == nums[i-1]) continue;
            // 计算2Sum
            ArrayList<List<Integer>> curr = twoSum(nums, i, 0 - nums[i]);
            res.addAll(curr);
        }
        return res;
    }
    
    private ArrayList<List<Integer>> twoSum(int[] nums, int i, int target){
        int left = i + 1, right = nums.length - 1;
        ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
        while(left < right){
            if(nums[left] + nums[right] == target){
                ArrayList<Integer> curr = new ArrayList<Integer>();
                curr.add(nums[i]);
                curr.add(nums[left]);
                curr.add(nums[right]);
                res.add(curr);
                do {
                    left++;
                }while(left < nums.length && nums[left] == nums[left-1]);
                do {
                    right--;
                } while(right >= 0 && nums[right] == nums[right+1]);
            } else if (nums[left] + nums[right] > target){
                right--;
            } else {
                left++;
            }
        }
        return res;
    }
}

Key

tips：很久没有写Java了，花了点时间去整理了一些知识，所以上面的算法其实是ctrl+v的，现在整理一下list相关的知识：

1.List<List>为嵌套的list集合，声明方式

List<List****> list = new Array***()*

or

List<List> list = new ArrayList<>();//recomend

2.List是一个接口，而ArrayList是List接口的一个实现类

List list = new List();//是错误的用法

List list = new ArrayList();//list会丢失ArrayList的trimToSize()方法

ArrayList list=newArrayList()

3.然后明天再回来重新写这道题