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/*
18. 4Sum
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]
]
*/
package twopointers;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class FourSum {
public static List<List<Integer>> fourSum(int[] nums, int target) {
ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
int len = nums.length;
if (nums == null || len < 4) {
return res;
}
Arrays.sort(nums);
return kSum(nums, target, 4, 0);
}
public static List<List<Integer>> kSum(int[] nums, int target, int k, int index) {
if (index >= nums.length) {
return null;
}
ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
if (k == 2) {
int i = index, j = nums.length - 1;
while(i < j) {
//find a pair
if(target - nums[i] == nums[j]) {
List<Integer> temp = new ArrayList<>();
temp.add(nums[i]);
temp.add(target-nums[i]);
res.add(temp);
//skip duplication
while(i<j && nums[i]==nums[i+1]) {
i++;
}
while(i<j && nums[j-1]==nums[j]) {
j--;
}
i++;
j--;
//move left bound
} else if (target - nums[i] > nums[j]) {
i++;
//move right bound
} else {
j--;
}
}
} else {
for (int i = index; i < nums.length - k + 1; i++) {
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
List<List<Integer>> list = kSum(nums, target - nums[i], k - 1, i + 1);
if (list != null && list.size() > 0) {
for (List<Integer> val : list) {
val.add(0, nums[i]);
}
res.addAll(list);
}
}
}
return res;
}
}