LeetCode – 4Sum (Java)

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.

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)

Java Solution

A typical k-sum problem. Time is N to the power of (k-1).

public List<List<Integer>> fourSum(int[] nums, int target) {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    if(nums==null|| nums.length<4)
        return result;
    for(int i=0; i<nums.length-3; i++){
        if(i!=0 && nums[i]==nums[i-1])
        for(int j=i+1; j<nums.length-2; j++){
            if(j!=i+1 && nums[j]==nums[j-1])
            int k=j+1;
            int l=nums.length-1;
                }else if(nums[i]+nums[j]+nums[k]+nums[l]>target){
                    List<Integer> t = new ArrayList<Integer>();
                    while(k<l &&nums[l]==nums[l+1] ){
                    while(k<l &&nums[k]==nums[k-1]){
    return result;
Category >> Algorithms  
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  • Matias SM

    I think you don’t need the Set to avoid duplicates if you check for each idx that the current value is different from the last one used. (same logic used in 3Sum solution)

  • Rahul Nakkanwar

    agreed…hashset is not required here

  • Renzo
  • Charles Gao

    You could simply check if the temp already exist by using result.contains(temp) instead of using HashSet.

  • Rohit

    For checking duplicate can we do this ?
    whenever we find the match get the sum of the i+j+k+l and store it in arraylist and everytime we find a match we can check if that sum exists in the arraylist if not its a new match or else already found.

  • Jinyao Xu

    why not give it a try?

  • ryanlr

    The array is sorted at the beginning.

  • ryanlr

    Good idea!


    Shouldn’t it be TreeSet or something instead of ArrayList. The order does matter while comparing one ArrayList to another.

  • Jinyao Xu

    One style optimization of your code is that you don’t need to use both hashset and result.
    You just need the hashset.
    Set<List> hashset = new HashSet();
    When you add tuple of 4, do this:
    hashset.add(Arrays.asLists(a, b, c, d)); // pay attention to the desc order

    Then, when you return, do this:
    return new ArrayList<List>(hashset);

    This makes your code clearer

  • CopperCash

    I guess that is because if the duplicate check is based on `while` and `index`, it will require 4 check points every loop. Since there is a `set` to make user there is no duplicate, so no need to check with `while`.

  • sammy

    Very nice solution! Can you explain why dont you use the same approach as in 3Sum, i.e. to skip repeating numbers with a while check?

  • anon

    There is a quadratic algorithm for this: http://stackoverflow.com/questions/14732277/quadratic-algorithm-for-4-sum (read first comment). I suppose that you can solve K-sum in O(N^((K+1)/2)) using the “store (K+1)/2-sums in a hash and then search for a match in the hash” logic.