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.

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)

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;
 
    Arrays.sort(nums);
 
    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 k=j+1;
            int l=nums.length-1;
            while(k<l){
                if(nums[i]+nums[j]+nums[k]+nums[l]<target){
                    k++;
                }else if(nums[i]+nums[j]+nums[k]+nums[l]>target){
                    l--;
                }else{
                    List<Integer> t = new ArrayList<Integer>();
                    t.add(nums[i]);
                    t.add(nums[j]);
                    t.add(nums[k]);
                    t.add(nums[l]);
                    result.add(t);
 
                    k++;
                    l--;
 
                    while(k<l &&nums[l]==nums[l+1] ){
                        l--;
                    }
 
                    while(k<l &&nums[k]==nums[k-1]){
                        k++;
                    }
                }
 
 
            }
        }
    }
 
    return result;
}