143. Reorder List

/**

143. Reorder List

https://leetcode.com/problems/reorder-list/description/

* @param {ListNode} head
* @return {void} Do not return anything, modify head in-place instead.
*/

var reorderList = function (head) {
　　if (head==null || head.next==null)
　　　　return;
　　//1.find the middle of list
　　let middle = findMiddle(head);
　　let secondHead = middle.next;
　　
　　middle.next = null;//断开中点和后一段
　　//2.reserve it
　　let reverseSecondHead = reverseLinkedList(secondHead);
　　//3.merge two list by items one by one
　　return mergeLinkedList(head,reverseSecondHead);
};

var findMiddle = function (head) {
　　let slow = head, fast = head;
　　while (fast != null && fast.next != null) {
　　　　slow = slow.next;
　　　　fast = fast.next.next;
　　}
　　return slow;
};

var reverseLinkedList = function (head) {
　　let prev = null;
　　let next = null;
　　let current = head;
　　while (current != null) {
　　　　next = current.next;
　　　　current.next = prev;//point to prve
　　　　prev = current;
　　　　current = next;
　　}
　　return prev;
};

var mergeLinkedList = function (node1,node2) {
　　//将第二个链表的元素间隔地插入第一个链表中
　　let cur = node1;
　　while (node2!=null){
　　　　let temp = node2.next;
　　　　node2.next = cur.next;
　　　　cur.next = node2;
　　　　cur = cur.next.next;
　　　　node2 = temp;
　　}
};