Some people are standing in a row in a park. There are trees between them which cannot be moved. Your task is to rearrange the people by their heights in a non-descending order without moving the trees. People can be very tall!

int[] sortByHeight(int[] a) {
    List<int> trees = new List<int>();
    List<int> people = new List<int>();
    for (int i = 0; i < a.Length; i++) 
    {
        if(a[i] == -1) trees.Add(i);
        else people.Add(a[i]);
    }
    people.Sort();
    int index = 0;
    foreach(int i in trees)
    {
        a[i] = -1;
    }
    foreach(int i in people)
    {
        a[index] = i;
        index++;
    }
    return a;
}