3Sum

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这里写图片描述

使用3层for循环暴力之

public class Solution {   public static  List<List<Integer>> threeSum(int[] nums) {        List<List<Integer>> countList = new ArrayList<List<Integer>>();        List<Integer> list = null;        int length = nums.length;        for(int i = 0;i<length;i++){            for(int j = i+1;j<length;j++){                for(int k = j+1;k<length;k++){                    if(nums[i]+nums[j]+nums[k]==0){                        list = new ArrayList<Integer>();                        int[] nu = new int[3];                        nu[0] = nums[i];                        nu[1] = nums[j];                        nu[2] = nums[k];                        Arrays.sort(nu);                        list.add(nu[0]);                        list.add(nu[1]);                        list.add(nu[2]);                        if(!countList.contains(list)){                            countList.add(list);                        }                       }                }            }        }        return countList;    }    public static void main(String[] args){        int[] nums = {-1,0,1,2,-1,-4};        List<List<Integer>> countList = threeSum(nums);        System.out.println(countList.size());        for(int i = 0;i < countList.size();i++){            System.out.println(countList.get(i));        }     }}

当输入参数为[13,14,1,2,-11,-11,-1,5,-1,-11,-9,-12,5,-3,-7,-4,-12,-9,8,-13,-8,2,-6,8,11,7,7,-6,8,-9,0,6,13,-14,-15,9,12,-9,-9,-4,-4,-3,-9,-14,9,-8,-11,13,-10,13,-15,-11,0,-14,5,-4,0,-3,-3,-7,-4,12,14,-14,5,7,10,-5,13,-14,-2,-6,-9,5,-12,7,4,-8,5,1,-10,-3,5,6,-9,-5,9,6,0,14,-15,11,11,6,4,-6,-10,-1,4,-11,-8,-13,-10,-2,-1,-7,-9,10,-7,3,-4,-2,8,-13]超时，没有想到更好的办法。

百度后，发现对数组进行递增排序，此时不需要对得到的结果集合进行递增排序，得到的结果集即为递增排序集合，节省了对结果集进行排序的时间

public class Solution {   public static  List<List<Integer>> threeSum(int[] nums) {         List<List<Integer>> countList = new ArrayList<List<Integer>>();        List<Integer> list = null;        int length = nums.length;        //对数组进行排序        Arrays.sort(nums);        for(int i = 0;i<length-2;i++){            //nums[i]=nums[i-1]            if(i >0 && nums[i] == nums[i-1]){                continue;            }            int p = i+1;            int q= length-1;            while(p<q){                //p!=q是为了避免nums[p]和nums[q]为相同索引下的值                if(nums[i]+nums[p]+nums[q]==0&&p!=q){                    list = new ArrayList<Integer>();                    list.add(nums[i]);                    list.add(nums[p]);                    list.add(nums[q]);                    countList.add(list);                    //nums[p] == nums[p+1]避免结果集重复                    while(p<q&&nums[p] == nums[p+1]){                        p++;                    }                    //nums[q]==nums[q-1]避免结果集重复                    while (p<q&&nums[q]==nums[q-1]) {                        q--;                    }                    p++;                    q--;                //nums[i]+nums[p]+nums[q]<0说明nums[p]的值较小，不能满足和为0，P的值需要增加                }else if(nums[i]+nums[p]+nums[q]<0){                    p++;                //nums[i]+nums[p]+nums[q]>0说明nums[q]的值较大，不能满足和为0，q的值需要减小                }else{                    q--;                }            }        }        return countList;    }    public static void main(String[] args){        int[] nums = {-1,0,1,2,-1,-4};        List<List<Integer>> countList = threeSum(nums);        System.out.println(countList.size());        for(int i = 0;i < countList.size();i++){            System.out.println(countList.get(i));        }     }}

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