Here is my code which pass all tests, but I saw a lot of check about n%(2*k)==0 and don't really understand why and I'm curious if someone would care to explain

public static List<Integer> absolutePermutation(int n, int k) {
// Write your code here
    Map<Integer,Integer> map = new TreeMap<>();
    for(int i=1;i<=n;i++){
        if(i-k>0&&!map.containsKey(i-k)){
            map.put(i-k,i);
        }else if(i+k<=n){
            map.put(i+k,i);
        } else{
            map = new TreeMap<>();
            map.put(1,-1);
            break;
        }
    }
    return new ArrayList<>(map.values());
}