4Sum
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.
For example, given array S = {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)
Analysis
-Similar to 3Sum, just need to add another 'for'
Code
public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>> res = new ArrayList<List<Integer>>();
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 low = j+1, high = nums.length-1;
while (low < high){
int sum = nums[i]+nums[j]+nums[low]+nums[high];
if (sum == target) {
List<Integer> list = new ArrayList<Integer>(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
res.add(list);
low++;
high--;
while (low < high && low<nums.length && nums[low-1]==nums[low]) {low++;}
while (low < high && high>0 && nums[high+1]==nums[high]) {high--;}
}
else if (sum < target) {low++;}
else {high--;}
}
}
}
return res;
}
}