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Solution.java
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Solution.java
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//
// DFS, Array, Two Pointers, Sorting(4Sum)
// https://leetcode.com/problems/4sum/
// Created by hyungwook on 2022/02/25.
//
import java.util.*;
class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
return kSum(nums, target, 0, 4);
}
public List<List<Integer>> kSum(int[] nums, int target, int start, int k) {
List<List<Integer>> res = new ArrayList<>();
if (start == nums.length) {
return res;
}
int average_value = target / k;
if (nums[start] > average_value || average_value > nums[nums.length - 1]) {
return res;
}
if (k == 2) {
return twoSum(nums, target, start);
}
for (int i = start; i < nums.length; ++i) {
if (i == start || nums[i - 1] != nums[i]) {
for (List<Integer> subset : kSum(nums, target - nums[i], i + 1, k - 1)) {
res.add(new ArrayList<>(Arrays.asList(nums[i])));
res.get(res.size() - 1).addAll(subset);
}
}
}
return res;
}
public List<List<Integer>> twoSum(int[] nums, int target, int start) {
List<List<Integer>> res = new ArrayList<>();
Set<Integer> s = new HashSet<>();
for (int i = start; i < nums.length; ++i) {
if (res.isEmpty() || res.get(res.size() - 1).get(1) != nums[i]) {
if (s.contains(target - nums[i])) {
res.add(Arrays.asList(target - nums[i], nums[i]));
}
}
s.add(nums[i]);
}
return res;
}
}