667. Beautiful Arrangement II

Given two integers n and k, construct a list answer that contains n different positive integers ranging from 1 to n and obeys the following requirement:

• Suppose this list is answer = [a1, a2, a3, … , an], then the list [|a1 - a2|, |a2 - a3|, |a3 - a4|, … , |an-1 - an|] has exactly k distinct integers.

Return the list answer. If there multiple valid answers, return any of them.

class Solution {
public:
    vector<int> constructArray(int n, int k) {
        vector<int> res{1};
        int l = 2, r = n;
        bool add = true;
        unordered_set<int> us{1};
        while(--k) {
            if(add) res.push_back(res.back() + k + 1);
            else res.push_back(res.back() - k - 1);
            add = !add;
            us.insert(res.back());
        }

        for(int i = 1; i <= n; i++) if(!us.count(i)) res.push_back(i);
        return res;
    }
};