Reference: LeetCode
Difficulty: Easy

My Post: My Java Solution (Sentinel, Circular, Doubly Linked List, Size)

Problem

Design your implementation of the linked list. You can choose to use the singly linked list or the doubly linked list. A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node. If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Example: 

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MyLinkedList linkedList = new MyLinkedList();
linkedList.addAtHead(1);
linkedList.addAtTail(3);
linkedList.addAtIndex(1, 2);  // linked list becomes 1->2->3
linkedList.get(1);            // returns 2
linkedList.deleteAtIndex(1);  // now the linked list is 1->3
linkedList.get(1);            // returns 3

Analysis

  • I use a sentinel and make the list circular.
  • I also add prev pointer to implement doubly linked list.

Test Case:

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["MyLinkedList","addAtHead","addAtTail","addAtIndex","get","deleteAtIndex","get"]
[[],[1],[3],[1,2],[1],[1],[1]]
[null,null,null,null,2,null,3]
["MyLinkedList","addAtHead","addAtTail","addAtIndex","get","deleteAtIndex","get"]
[[],[1],[3],[1,2],[-1],[1],[-3]]
[null,null,null,null,-1,null,-1]
["MyLinkedList","addAtIndex","get","deleteAtIndex"]
[[],[-1,0],[0],[-1]]
[null,null,0,null] // this test case is not expected

One of the test case is wrong. The judge assumes that addAtIndex returns the first element when the input is negative.

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class MyLinkedList {
  /** ListNode class */
  private class ListNode { // Doubly Linked List
    int val;
    ListNode next;
    ListNode prev;
    
    ListNode(int v) { 
      this(v, null, null); 
    }
    ListNode(int v, ListNode p, ListNode n) {
      val = v; prev = p; next = n;
    }
  }
  
  private ListNode sentinel;
  private int size;
  
  public MyLinkedList() {
    size = 0;
    sentinel = new ListNode(-1);
    sentinel.next = sentinel; // circular linked list
    sentinel.prev = sentinel;
  }

  /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
  public int get(int index) {
    if (isIndexValid(index) == false || size == 0) {
      return -1;
    } // index is valid
    ListNode p = sentinel.next;
    while (index > 0) {
      p = p.next;
      --index;
    }
    return p.val;
  }

  /** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */
  public void addAtHead(int val) {
    // consider size = 0, 1
    // sentinel -> p1 -> p2
    // sentinel -> newNode -> p1 -> p2
    ListNode newNode = new ListNode(val, sentinel, sentinel.next);
    sentinel.next.prev = newNode;
    sentinel.next = newNode;
    ++size;
  }

  /** Append a node of value val to the last element of the linked list. */
  public void addAtTail(int val) {
    // consider size = 0, 1
    ListNode newNode = new ListNode(val, sentinel.prev, sentinel);
    sentinel.prev.next = newNode;
    sentinel.prev = newNode;
    ++size;
  }

  /** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
  public void addAtIndex(int index, int val) {
    if (index < 0) addAtHead(val); // Test case bug
    if (index < 0 || index > size) { // can't use isIndexValid
      return;
    }
    ListNode prev = sentinel; // consider size = 0, 1, 2
    ListNode p = sentinel.next;
    while (index > 0) {
      prev = p;
      p = p.next;
      --index;
    }
    ListNode newNode = new ListNode(val, prev, prev.next);
    prev.next.prev = newNode;
    prev.next = newNode;
    ++size;
  }

  /** Delete the index-th node in the linked list, if the index is valid. */
  public void deleteAtIndex(int index) {
    if (isIndexValid(index) == false || size == 0) {
      return;
    }
    ListNode prev = sentinel; // consider size = 1, 2, 3
    ListNode p = sentinel.next;
    while (index > 0) {
      prev = p;
      p = p.next;
      --index;
    }
    prev.next = p.next; // delete p
    p.next.prev = prev;
    --size;
  }
  
  private boolean isIndexValid(int index) {
    return !(index < 0 || index > size - 1);
  }
}