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54 lines (45 loc) · 2.07 KB
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// 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.
// The solution set must not contain duplicate quadruplets.
// See: https://leetcode.com/problems/4sum/
package leetcode.array_and_hashtable;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class Sum4 {
// O(n.log(n) + n^3) time complexity, O(1) space (without the output list)
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>> quadruplets = new ArrayList<>();
for (int i = 0; i < nums.length - 3; i++) {
if (i > 0 && nums[i] == nums[i - 1]) continue;
for (int j = i + 1; j < nums.length - 2; j++) {
if (j > i + 1 && nums[j] == nums[j - 1]) continue;
int left = j + 1;
int right = nums.length - 1;
while (left < right) {
int sum = nums[i] + nums[j] + nums[left] + nums[right];
if (sum == target) {
quadruplets.add(Arrays.asList(
new Integer[] { nums[i], nums[j], nums[left], nums[right] }));
while (left < right && nums[left] == nums[left + 1]) left++;
while (left < right && nums[right] == nums[right - 1]) right--;
left++;
right--;
} else if (sum < target)
left++;
else
right--;
}
}
}
return quadruplets;
}
public static void main(String[] args) {
Sum4 sln = new Sum4();
System.out.println(sln.fourSum(new int[] { 1, 0, -1, 0, -2, 2 }, 0));
System.out.println(sln.fourSum(new int[] { -3, -2, -1, 0, 0, 1, 2, 3 }, 0));
System.out.println(sln.fourSum(new int[] { 5, 5, 3, 5, 1, -5, 1, -2 }, 4));
}
}