Absolute Permutation

• Time : O(n)
• Space : O(n)

vector<int> absolutePermutation(int n, int k) {
    vector<int> res;
    unordered_set<int> us;
    for(int i = 1; i <= n; i++) {
        if(i-k > 0 and !us.count(i-k)) res.push_back(i - k);
        else res.push_back(i + k);
        if(us.count(res.back())) return {-1};
        if(res.back() > n) return {-1};
        us.insert(res.back());
    }
    return res;
}