reorder-list.js

/**You are given the head of a singly linked-list. The list can be represented as:

L0 → L1 → … → Ln - 1 → Ln
Reorder the list to be on the following form:

L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …
You may not modify the values in the list's nodes. Only nodes themselves may be changed. */
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {void} Do not return anything, modify head in-place instead.
 */
 var reorderList = function(head) {
    if (head === null) return null;
    
    let slow = head;
    let fast = head;
    
    while (fast && fast.next) {
      slow = slow.next;
      fast = fast.next.next;
    }
    
    let firstHead = head;
    let secondHead = reverseList(slow);
    
    while (secondHead) {
      let temp1 = firstHead.next;
      let temp2 = secondHead.next;
      
      firstHead.next = secondHead;
      secondHead.next = temp1;
      
      firstHead = temp1;
      secondHead = temp2;
    }
    
    if (firstHead) {
      firstHead.next = null;
    }
    
    return head;
  };
  
  
  function reverseList(node) {
    let prev = null;
    let curr = node;
    while (curr) {
      let next = curr.next;
      curr.next = prev;
      prev = curr;
      curr = next;
    }
    
    return prev;
    
  }