1) Design Linked List

Hi geeks, in this article we going to design the Linked List with add at head, add at tail, insert at specific position and delete operations. This is one of the good question from Leetcode to test the LinkedList knowledge.

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 nextval 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.

Implement these functions in your linked list class:

  • get(index) : Get the value of the index-th node in the linked list. If the index is invalid, return -1.
  • addAtHead(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.
  • addAtTail(val) : Append a node of value val to the last element of the linked list.
  • addAtIndex(index, val) : 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.
  • deleteAtIndex(index) : Delete the index-th node in the linked list, if the index is valid.
Input: 
["MyLinkedList","addAtHead","addAtTail","addAtIndex","get","deleteAtIndex","get"]
[[],[1],[3],[1,2],[1],[1],[1]]
Output:  
[null,null,null,null,2,null,3] 

Explanation: MyLinkedList linkedList = new MyLinkedList(); // Initialize empty LinkedList 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

Solution:

For this solution, I used a singly linked list with a sentinel node. The sentinel node is a nice way of avoiding edge cases. It makes the code much simpler. I also keep a pointer to the last element (tailPred) to allow fast insertion at the tail.

To avoid redundancy, I added two helper methods, findPred and addAfter. This keeps the code clean.

/**
 * A singly linked list with a left sentinel node.
 */
class MyLinkedList {

	/** A very simple node class. */
	private static class Node {
		int val;
		Node next;
	}

	// Predecessor of the first element
	private Node headPred;
	// Predecessor of the tail
	private Node tailPred;
	private int length;

	/** Initialize your data structure here. */
	public MyLinkedList() {
		headPred = new Node();
		tailPred = headPred;
		length = 0;
	}

	/** 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 ((index < 0) || (index >= length)) {
			return -1;
		}
		return findPred(index).next.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) {
		if (length == 0) {
			addAtTail(val);
		} else {
			addAfter(headPred, val);
		}
	}

	/** Append a node of value val to the last element of the linked list. */
	public void addAtTail(int val) {
		addAfter(tailPred, val);
		tailPred = tailPred.next;
	}

	/** 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);
		} else if (index == length) {
			addAtTail(val);
		} else if ((index >= 0) && (index < length)) {
			addAfter(findPred(index), val);
		}
	}

	/** Delete the index-th node in the linked list, if the index is valid. */
	public void deleteAtIndex(int index) {
		if ((index >= 0) && (index < length)) {
			Node pred = findPred(index);
			if (index == length - 1) { // Remove last element
				// Move tail to the left
				tailPred = pred;
			}
			pred.next = pred.next.next;
			--length;
		}
	}

	/** Return the predecessor of the index-th node. */
	private Node findPred(int index) {
		Node pred = headPred;
		for (int i = 0; i < index; ++i) {
			pred = pred.next;
		}
		return pred;
	}

	/** Add an element after the given node. */
	private void addAfter(Node pred, int val) {
		Node node = new Node();
		node.val = val;
		node.next = pred.next;
		pred.next = node;
		++length;
	}
}

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