花花酱 LeetCode 2829. Determine the Minimum Sum of a k-avoiding Array

You are given two integers, n and k.

An array of distinct positive integers is called a k-avoiding array if there does not exist any pair of distinct elements that sum to k.

Return the minimum possible sum of a k-avoiding array of length n.

Example 1:

Input: n = 5, k = 4
Output: 18
Explanation: Consider the k-avoiding array [1,2,4,5,6], which has a sum of 18.
It can be proven that there is no k-avoiding array with a sum less than 18.

Example 2:

Input: n = 2, k = 6
Output: 3
Explanation: We can construct the array [1,2], which has a sum of 3.
It can be proven that there is no k-avoiding array with a sum less than 3.

Constraints:

• 1 <= n, k <= 50

Solution 1: Greedy + HashTable

Always choose the smallest possible number starting from 1.

Add all chosen numbers into a hashtable. For a new candidate i, check whether k – i is already in the hashtable.

Time complexity: O(n)
Space complexity: O(n)