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Problem solution · Java

Design Skiplist

Design Skiplist: a Java solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
64 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Design Skiplist, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 64 lines of Java from the credited upstream file 1206.java.
  • The implementation visibly relies on work queue.
  • 5 loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeDesign Skiplist · JavaJava
Use this to learn the idea, then write your own version.
class Node {  public int val;  public Node next;  public Node down;  public Node(int val, Node next, Node down) {    this.val = val;    this.next = next;    this.down = down;  }} class Skiplist {  public boolean search(int target) {    for (Node node = dummy; node != null; node = node.down) {      node = advance(node, target);      if (node.next != null && node.next.val == target)        return true;    }    return false;  }   public void add(int num) {    // Collect nodes that are before the insertion point.    Deque<Node> nodes = new ArrayDeque<>();    for (Node node = dummy; node != null; node = node.down) {      node = advance(node, num);      nodes.push(node);    }     Node down = null;    boolean shouldInsert = true;    while (shouldInsert && !nodes.isEmpty()) {      Node prev = nodes.poll();      prev.next = new Node(num, prev.next, down);      down = prev.next;      shouldInsert = Math.random() < 0.5;    }     // Create a topmost new level dummy that points to the existing dummy.    if (shouldInsert)      dummy = new Node(-1, null, dummy);  }   public boolean erase(int num) {    boolean found = false;    for (Node node = dummy; node != null; node = node.down) {      node = advance(node, num);      if (node.next != null && node.next.val == num) {        node.next = node.next.next;        found = true;      }    }    return found;  }   private Node dummy = new Node(-1, null, null);   private Node advance(Node node, int target) {    while (node.next != null && node.next.val < target)      node = node.next;    return node;  }}

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