四数之和
给你一个由 n 个整数组成的数组 nums ，和一个目标值 target 。请你找出并返回满足下述全部条件且不重复的四元组 [nums[a], nums[b], nums[c], nums[d]] ：
0 <= a, b, c, d < n
a、b、c 和 d 互不相同
nums[a] + nums[b] + nums[c] + nums[d] == target
你可以按 任意顺序 返回答案 。
示例 1：
输入：nums = [1,0,-1,0,-2,2], target = 0
输出：[[-2,-1,1,2],[-2,0,0,2],[-1,0,0,1]]
示例 2：
-109 <= nums[i] <= 109
-109 <= target <= 109
package com.kk;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
//18 四数之和
public class FourSum {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int[] nums = new int[6];
for (int i = 0; i < 6; i++) {
nums[i]=scanner.nextInt();
}
int target=scanner.nextInt();
List<List<Integer>> lists = fourSum(nums, target);
System.out.println(lists);
}
public static List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> quadruplets = new ArrayList<List<Integer>>();
if (nums==null||nums.length<4){
return quadruplets;
}
Arrays.sort(nums);
int length=nums.length;
for (int i = 0; i < length-3; i++) {
if (i>0&&nums[i]==nums[i-1]){
continue;
}
if (nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target){
break;
}
if (nums[i]+nums[length-3]+nums[length-2]+nums[length-1]<target){
continue;
}
for (int j = i+1; j < length - 2; j++) {
if(j>i+1&&nums[j]==nums[j-1]){
continue;
}
if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target){
break;
}
if (nums[i]+nums[j]+nums[length-2]+nums[length-1]<target){
continue;
}
int left=j+1,right=length-1;
while(left<right){
long sum=nums[i]+nums[j]+nums[left]+nums[right];
if (sum==target){
quadruplets.add(Arrays.asList(nums[i],nums[j],nums[left],nums[right]));
while (left<right&&nums[left]==nums[left+1]){
left++;
}
left++;
while (left<right&&nums[right]==nums[right-1]){
right--;
}
right--;
}else if(sum<target){
left++;
}else{
right--;
}
}
}
}
return quadruplets;
}
}
以上为官方给出的答案，官方新给出了一个示例：[0,0,0,1000000000,1000000000,1000000000,1000000000] 1000000000，这个是测试通过的
因此使用以下方法，不过效率较低
class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> res = new ArrayList<>();
if(nums == null || nums.length < 4) return res;
Arrays.sort(nums);
int len = nums.length;
for(int i = 0; i < len - 3; i++){
if(i > 0 && nums[i - 1] == nums[i]) continue;
for(int j = i + 1; j < len - 2; j++){
if(j > i + 1 && nums[j - 1] == nums[j]) continue;
int L = j + 1;
int R = len - 1;
while(L < R){
int sum = nums[i] + nums[j] + nums[L] + nums[R];
if(sum == target){
res.add(Arrays.asList(nums[i],nums[j],nums[L],nums[R]));
while(L < R && nums[L + 1] == nums[L]) L++;
while(L < R && nums[R - 1] == nums[R]) R--;
L++;
R--;
}else if(sum > target){
R--;
}else{
L++;
}
}
}
}
return res;
}
}