Leecode no.18 四数之和

package com.example.demo.leecode;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * 四数之和等于目标
 * @Date 2020/12/8
 * @author Tang
 *
 * 给定一个包含 n 个整数的数组 nums 和一个目标值 target，判断 nums 中是否存在四个元素 a，b，c 和 d ，
 * 使得 a + b + c + d 的值与 target 相等？找出所有满足条件且不重复的四元组。
 * 注意：答案中不可以包含重复的四元组。
 */
public class FourSumTarget {
    private Set<List<Integer>> resultSet = new HashSet<>();

    public List<List<Integer>> execute(int[] nums, int target){
        if(nums.length < 4){
            return new ArrayList<>();
        }
        nums = sort(nums, 0, nums.length - 1);
        //外层循环获取第一位数 a
        for(int i = 0; i < nums.length - 3; i++){
            int a = nums[i];

            //内层循环第二位数字 b
            for (int j = i + 1; j < nums.length - 1; j++) {
                int b = nums[j];

                int index1 = j + 1;
                int index2 = nums.length - 1;

                int c = nums[index1];
                int d = nums[index2];
                while(index1 < index2){
                    if(a + b + c + d > target){
                        index2--;
                        d = nums[index2];
                    }else if(a + b + c + d < target){
                        index1++;
                        c = nums[index1];
                    }else{
                        //成功相等
                        setResultList(a, b, c, d);
                        index2--;
                        d = nums[index2];
                    }

                }
            }

        }
        List<List<Integer>> resultList = new ArrayList<>();
        resultList.addAll(resultSet);

        return resultList;

    }

    private void setResultList(int a, int b, int c, int d){
        List<Integer> result = new ArrayList<>();
        result.add(a);
        result.add(b);
        result.add(c);
        result.add(d);
        resultSet.add(result);
    }

    /**
     * 快排
     */
    private int[] sort(int[] nums, int left, int right){
        if(left >= right){
            return nums;
        }
        int i = left;
        int j = right;
        int base = nums[i];

        while(i < j){
            int temp = 0;
            while(nums[j] > base){
                j--;
            }
            if(i < j){
                temp = nums[j];
                nums[j] = nums[i];
                nums[i] = temp;
                i++;
            }
            while(nums[i] < base){
                i++;
            }
            if(i < j){
                temp = nums[i];
                nums[i] = nums[j];
                nums[j] = temp;
                j--;
            }
        }
        nums[i] = base;

        int[] sortLeft = sort(nums, left, i - 1);
        int[] sortRight = sort(sortLeft, i +1, right);
        return sortRight;
    }

    public static void main(String[] args) {
        int[] nums = {6,4,8,7,5,9,3,1,6,6,9,1};
        int[] nums2 = {1,0,-1,0,-2,2};
        int[] nums3 = {0,0,0,0};
        List<List<Integer>> execute = new FourSumTarget().execute(nums3, 0);
        for (List<Integer> i: execute) {
            System.out.print(i);
        }
    }

}