18. 4Sum (Medium)
https://leetcode.com/problems/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] ]
Solutions
class Solution {
// Similar to 3 sum, just add another outer loop
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> ans = new ArrayList<>();
if (nums.length < 4) {
return ans;
}
Arrays.sort(nums);
for (int i = 0; i <= nums.length - 4; i++) {
if (i != 0 && nums[i] == nums[i - 1]) {
continue; // avoid duplicates
}
for (int j = i + 1; j <= nums.length - 3; j++) {
if (j != i + 1 && nums[j] == nums[j - 1]) {
continue; // avoid duplicates
}
int x = j + 1;
int y = nums.length - 1;
while (x < y) {
if (x != j + 1 && nums[x] == nums[x - 1]) { // avoid duplicates
x++;
continue;
}
if (y != nums.length - 1 && nums[y] == nums[y + 1]) { // avoid duplicates
y--;
continue;
}
int sum = nums[i] + nums[j] + nums[x] + nums[y];
if (sum < target) {
x++;
}
if (sum > target) {
y--;
}
if (sum == target) {
ans.add(Arrays.asList(nums[i], nums[j], nums[x++], nums[y--]));
}
}
}
}
return ans;
}
}