Detect circular reference of linked list in Javascript.

circular-reference-detect.js

/*
 * circular-reference-detect.js
 * author: Hiroshi Kori 
 *
 * Detect circular reference of linked list
 * 
 * 1) Overview
 *  - Reverse linked-list until finding a node whose next pointer is null.
 *  - Chekck the head node if its next pointer is null
 *    1) null:      the node is not traversed twice. (no circular reference)
 *    2) otherwise: traversed twice because of circulation. (circular reference)
 * 
 * 2) Usage Example
 * 
 * var head={val:"head"},n1={val:"1"},n2={val:"2"},n3={val:"3"},n4={val:"4"};
 * 
 * var cir_linked_list_helper = function(head) {
 *   head.next=n2,n2.next=n4,n4.next=n3,n3.next=n1,n1.next=n4;
 *   return head;
 * };
 * var non_cir_linked_list_helper = function(head) {
 *   head.next=n2,n2.next=n4,n4.next=n3,n3.next=n1;
 *   return head;
 * };
 * 
 * head=cir_linked_list_helper(head);
 * console.log(is_circular(head));
 * 
 */ 

var is_circular = (function() {

  var reverse = function(head){
    var current=head,next=null,prev=null;
    while(current.next!=null && current.next!=undefined) {
      next=current.next;
      current.next=prev;
      prev=current;
      current=next;
    }
    current.next=prev;
    return current;
  };
  return function(head){
    var tested=head.next;
    reverse(tested);
    return tested.next!==null;
  };
})(head);