4.2 $\mathtt{SkiplistSSet}$: An Efficient $\mathtt{SSet}$ Implementation

A $\mathtt{SkiplistSSet}$ uses a skiplist structure to implement the $\mathtt{SSet}$ interface. When used this way, the list $L_0$ stores the elements of the $\mathtt{SSet}$ in sorted order. The $\mathtt{find(x)}$ method works by following the search path for the smallest value $\mathtt{y}$ such that $\ensuremath{\mathtt{y}}\ge\ensuremath{\mathtt{x}}$:

  Node* findPredNode(T x) {
    Node *u = sentinel;
    int r = h;
    while (r >= 0) {
      while (  u->next[r] != NULL && compare(u->next[r]->x, x) < 0)
        u = u->next[r]; // go right in list r
      r--; // go down into list r-1
    }
    return u;
  }
  T find(T x) {
    Node *u = findPredNode(x);
    return u->next[0] == NULL ? NULL : u->next[0]->x;
  }

Following the search path for $\mathtt{y}$ is easy: when situated at some node, $\mathtt{u}$, in $L_{\ensuremath{\mathtt{r}}}$, we look right to $\mathtt{u.next[r].x}$. If $\ensuremath{\mathtt{x}}>\ensuremath{\mathtt{u.next[r].x}}$, then we take a step to the right in $L_{\ensuremath{\mathtt{r}}}$, otherwise we move down into $L_{\ensuremath{\mathtt{r}}-1}$. Each step (right or down) in this search takes only constant time so, by Lemma 4.1, the expected running time of $\mathtt{find(x)}$ is $O(\log \ensuremath{\mathtt{n}})$.

Before we can add an element to a $\mathtt{SkipListSSet}$, we need a method to simulate tossing coins to determine the height, $\mathtt{k}$, of a new node. We do this by picking a random integer, $\mathtt{z}$, and counting the number of trailing $1$s in the binary representation of $\mathtt{z}$:¹

  int pickHeight() {
    int z = rand();
    int k = 0;
    int m = 1;
    while ((z & m) != 0) {
      k++;
      m <<= 1;
    }
    return k;
  }

To implement the $\mathtt{add(x)}$ method in a $\mathtt{SkiplistSSet}$ we search for $\mathtt{x}$ and then splice $\mathtt{x}$ into a few lists $L_0$,..., $L_{\ensuremath{\mathtt{k}}}$, where $\mathtt{k}$ is selected using the $\mathtt{pickHeight()}$ method. The easiest way to do this is to use an array, $\mathtt{stack}$, that keeps track of the nodes at which the search path goes down from some list $L_{\ensuremath{\mathtt{r}}}$ into $L_{\ensuremath{\mathtt{r}}-1}$. More precisely, $\mathtt{stack[r]}$ is the node in $L_{\ensuremath{\mathtt{r}}}$ where the search path proceeded down into $L_{\ensuremath{\mathtt{r}}-1}$. The nodes that we modify to insert $\mathtt{x}$ are precisely the nodes $\ensuremath{\mathtt{stack[0]}},\ldots,\ensuremath{\mathtt{stack[k]}}$. The following code implements this algorithm for $\mathtt{add(x)}$:

  bool add(T x) {
    Node *u = sentinel;
    int r = h;
    int comp = 0;
    while (r >= 0) {
      while (u->next[r] != NULL && (comp = compare(u->next[r]->x, x)) < 0)
        u = u->next[r];
      if (u->next[r] != NULL && comp == 0)
        return false;
      stack[r--] = u; // going down, store u
    }
    Node *w = newNode(x, pickHeight());
    while (h < w->height)
      stack[++h] = sentinel; // increasing height of skiplist
    for (int i = 0; i < w->height; i++) {
      w->next[i] = stack[i]->next[i];
      stack[i]->next[i] = w;
    }
    n++;
    return true;
  }

Figure 4.3: Adding the node containing $3.5$ to a skiplist. The nodes stored in $\mathtt{stack}$ are highlighted.

Removing an element, $\mathtt{x}$, is done in a similar way, except that there is no need for $\mathtt{stack}$ to keep track of the search path. The removal can be done as we are following the search path. We search for $\mathtt{x}$ and each time the search moves downward from a node $\mathtt{u}$, we check if $\ensuremath{\mathtt{u.next.x}}=\ensuremath{\mathtt{x}}$ and if so, we splice $\mathtt{u}$ out of the list:

  bool remove(T x) {
    bool removed = false;
    Node *u = sentinel, *del;
    int r = h;
    int comp = 0;
    while (r >= 0) {
      while (u->next[r] != NULL && (comp = compare(u->next[r]->x, x)) < 0) {
        u = u->next[r];
      }
      if (u->next[r] != NULL && comp == 0) {
        removed = true;
        del = u->next[r];
        u->next[r] = u->next[r]->next[r];
        if (u == sentinel && u->next[r] == NULL)
          h--; // skiplist height has gone down
      }
      r--;
    }
    if (removed) {
      delete del;
      n--;
    }
    return removed;
  }

Figure 4.4: Removing the node containing $3$ from a skiplist.

4.2.1 Summary

The following theorem summarizes the performance of skiplists when used to implement sorted sets:

Theorem 4..1   A $\mathtt{SkiplistSSet}$ implements the $\mathtt{SSet}$ interface. A $\mathtt{SkiplistSSet}$ supports the operations $\mathtt{add(x)}$, $\mathtt{remove(x)}$, and $\mathtt{find(x)}$ in $O(\log \ensuremath{\mathtt{n}})$ expected time per operation.

Footnotes

...:¹

This method does not exactly replicate the coin-tossing experiment since the value of $\mathtt{k}$ will always be less than the number of bits in an $\mathtt{int}$. However, this will have negligible impact unless the number of elements in the structure is much greater than $2^{32}=4294967296$.