題目大意:給定一個包含 n 個整數的數組 nums 和一個目標值 target,判斷 nums 中是否存在四個元素 a,b,c 和 d ,使得 a + b + c +d 的值與 target 相等?找出所有滿足條件且不重複的四元組。
注意:
答案中不可以包含重複的四元組。
Given an array nums
of n integers and an integer target
, are there elements a, b, c, and d in nums
such that a + b + c + d = target
? Find all unique quadruplets in the array which gives the sum of target
.
Note:
The solution set must not contain duplicate quadruplets.
Example:
Given array nums = [1, 0, -1, 0, -2, 2], and target = 0. A solution set is: [ [-1, 0, 0, 1], [-2, -1, 1, 2], [-2, 0, 0, 2] ]
示例:
給定數組 nums = [1, 0, -1, 0, -2, 2],和 target = 0。
滿足要求的四元組集合爲:
[
[-1, 0, 0, 1],
[-2, -1, 1, 2],
[-2, 0, 0, 2]
]
此題與15題求三數之和思路是一樣,所以可以轉化爲求三數之和的問題,降低複雜度。
package pers.leetcode;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
/**
* 4Sum 四數之和
* 難易程度:Medium
*
* @author jinghui.zhu
* @date 2019/5/17 9:31
*/
public class LeetCode18 {
public static void main(String[] args) {
int[] nums = {-3, -2, -1, 0, 0, 1, 2, 3};
int target = 0;
System.out.println("leetcode18:" + fourSum(nums, target));
}
public static List<List<Integer>> fourSum(int[] nums, int target){
//原始數組排序
Arrays.sort(nums);
List<List<Integer>> result = new ArrayList<>();
for (int i = 0; i < nums.length - 3; i++){
// 去重
if (i >= 1 && nums[i] == nums[i-1]){
continue;
}
int threeSum = target - nums[i];
for (int j = i + 1; j < nums.length - 2; j++){
//去重
if (j-1 != i && nums[j] == nums[j-1]){
continue;
}
int k = j + 1;
int z = nums.length - 1;
while (k < z){
if (nums[j] + nums[k] + nums[z] > threeSum){
z--;
}else if (nums[j] + nums[k] + nums[z] < threeSum){
k++;
}else {
List<Integer> temp = new ArrayList<>();
temp.add(nums[i]);
temp.add(nums[j]);
temp.add(nums[k]);
temp.add(nums[z]);
result.add(temp);
z--;
k++;
//去重
while (nums[k] == nums[k-1] && k < nums.length - 1){
k++;
}
//去重
while (nums[z] == nums[z+1] && z > 0){
z--;
}
}
}
}
}
return result;
}
}