nth To Last from LinkedList - Node.js Data Structure

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Description

nth To Last from LinkedList

Demo Code


function LinkedList() {
  this.head = null;/*from w w w . j a va  2s.c o  m*/
  this.tail = null;
}

function Node(data) {
  this.data = data;
  this.next = null;
  this.previous = null;
}

LinkedList.prototype.addToHead = function(el) {
  if (el) {
      var newNode = new Node(el);
    if(this.head === null){
      this.tail = this.head = newNode;
    }else{
      newNode.previous = null;
      this.head.previous = newNode;
      newNode.next = this.head;
      this.head = newNode;
    }
  } else {
    return undefined;
  }
}

function nthToLast(sll, k){
    //handle, 0 or negative value of k
    if (k <= 0) 
        return null;
    
    // Create two pointers both point to head
    var pointer1 = sll.head;
    var pointer2 = sll.head;
    
    // Move pointer2 forward k nodes into the list.
    for (var i = 0; i < k - 1; i ++) {
        if (pointer2 === null) return null;
        pointer2 = pointer2.next;
    }

    if (pointer2 === null) return null;

    // Move pointer1 and pointer 2 at the same speed.
    // Once the pointer2 reaches the end,
    // the pointer1 will be the kth last element
    while (pointer2.next !== null) {
        pointer1 = pointer1.next;
        pointer2 = pointer2.next;
    }
    
   return pointer1;
}

//create the LinkedList
var sll = new LinkedList();
sll.addToHead(5);
sll.addToHead(4);
sll.addToHead(3);
sll.addToHead(2);
sll.addToHead(1);

console.log('sll ', sll);
console.log(nthToLast(sll, 1)); //Object {data: 5, next: null}
console.log(nthToLast(sll, 2)); //Object {data: 4, next: Object}
console.log(nthToLast(sll, 3)); //Object {data: 3, next: Object}

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