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this is related to the following question:

Generalised 3SUM (k-SUM) problem?

Without loss of generality, let's only consider even $k$, or just $k=4$.

My question is, after summing all pairs of numbers, is it necessary to sort the list of sums? I understand we could use two pointers from left and right to sandwich the two pairs in $O(n^2)$ time, but the sorting requires $O(n^2\log(n))$ time.

If we use a hashmap to store the sums as key and their corresponding index pairs as value, then all operations can run in $O(n^2)$ time.

Am I missing something in that post or is it true for even $k$, $k$-sum can run in $O(n^{k/2})$ time?

Thanks!

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  • $\begingroup$ How would you sandwich with two pointers without sorting? $\endgroup$
    – Navjot Singh
    Sep 12, 2018 at 19:09
  • $\begingroup$ Let s = $n^2$, then time needed to sort is $O(slogs)$ which is not equal to $O(n^2logn)$. $\endgroup$
    – Navjot Singh
    Sep 12, 2018 at 19:11
  • $\begingroup$ Hashmap cannot store duplicates. So two pairs having same sum cannot be placed in the hashmap if sum is used as a key. Consider (1,3) and (2,2). $\endgroup$
    – Navjot Singh
    Sep 12, 2018 at 19:13
  • $\begingroup$ @JotWaraich $O(n^2 \log(n^2))=O(n^2 2\log(n))=O(n^2 \log(n))$ $\endgroup$
    – Kaa1el
    Sep 12, 2018 at 20:11
  • $\begingroup$ @JotWaraich in this case, either you don't need to store the duplicates (decision problem or return one such sum), or you can store in the value a list which consists of all such pairs (return all different sums). Either way, the complexity is not affected. $\endgroup$
    – Kaa1el
    Sep 12, 2018 at 20:13

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You are right, but Jeff's answer in the link you provided works in the "linear decision tree model". You cannot use hashing in that model.

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