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;
    }
}