delete-middle-node-of-linked-list.js

/* Delete the Middle Node of a Linked List

You are given the head of a linked list. Delete the middle node,
and return the head of the modified linked list.

The middle node of a linked list of size n is the ⌊n / 2⌋th node from the start using 0-based indexing,
where ⌊x⌋ denotes the largest integer less than or equal to x.
For n = 1, 2, 3, 4, and 5, the middle nodes are 0, 1, 1, 2, and 2, respectively.
 
Example 1:
Input: head = [1,3,4,7,1,2,6]
Output: [1,3,4,1,2,6]
Explanation:
The above figure represents the given linked list. The indices of the nodes are written below.
Since n = 7, node 3 with value 7 is the middle node, which is marked in red.
We return the new list after removing this node. 

Example 2:
Input: head = [1,2,3,4]
Output: [1,2,4]
Explanation:
The above figure represents the given linked list.
For n = 4, node 2 with value 3 is the middle node, which is marked in red.

Example 3:
Input: head = [2,1]
Output: [2]
Explanation:
The above figure represents the given linked list.
For n = 2, node 1 with value 1 is the middle node, which is marked in red.
Node 0 with value 2 is the only node remaining after removing node 1.
 

Constraints:
The number of nodes in the list is in the range [1, 105].
1 <= Node.val <= 105 */


/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */

/** optimal solution 
 * @param {ListNode} head
 * @return {ListNode}
 */
 var deleteMiddle = function(head) {
    let slow = head, fast = head;
    let prev = null;
    
    if(head === null || head.next === null){
        return null
    }
    
    while(slow && fast && fast.next){
        prev = slow;
        slow = slow.next;
        fast = fast.next.next;
    }
        
    prev.next = slow.next;
    slow.next = null;
    
    return head;
};

/** naive approach
 * @param {ListNode} head
 * @return {ListNode}
 */
 var deleteMiddle = function(head) {
    let nos = 0;
    
    let counterHead = head;
   
    while(counterHead){
        counterHead = counterHead.next;
        nos++;
    }
    
    if(nos <= 1){
        return null;
    }
    
    if(nos === 2){
        head.next = null;
        return head;
    }
    
    let prev = null;
    let temp = head;
    nos = Math.floor(nos / 2);
    
    while(temp && nos > 0){
        prev = temp;
        temp = temp.next;
        nos--;    
    }
    
    let nextNode = temp.next;
    prev.next = nextNode;
    temp.next = null;
    
    return head;
};