reverseLinkedList.js

/*
 * Reverse a singly linked list.
 * 
 * Example:
 * 
 * Input: 1->2->3->4->5->NULL
 * Output: 5->4->3->2->1->NULL
 * Follow up:
 * 
 * A linked list can be reversed either iteratively or recursively. Could you implement both?
*/

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function(head) {
    if (!head || !head.next){
        return head;
    }
    
    var reverseNode = {};
    var counter = 0;
    function traverse(node, reverseNode, counter){        
        if (!node.next){
            var finalReverseNode = new ListNode(node.val);
            finalReverseNode.next = reverseNode;
            return finalReverseNode;
        }
        
        var newReverseNode = new ListNode(node.val);

        if (counter === 0){
            newReverseNode.next = null;
        } else {
            newReverseNode.next = reverseNode;
        }
        
        counter++;
        return traverse(node.next, newReverseNode, counter);
    }
    
    return traverse(head, reverseNode, counter);
};