[路飞]leetcode-148.排序链表

给你链表的头结点 head ，请将其按 升序 排列并返回 排序后的链表 。力扣原文

示例 1：

输入： head = [4,2,1,3]
输出： [1,2,3,4]

示例 2：

输入： head = [-1,5,3,4,0]
输出： [-1,0,3,4,5]

示例 3：

输入： head = []
输出： []

解题：归并排序

var marge = function (head1, head2) {
  const dummyHead = new ListNode(0);
  let temp = dummyHead,
    temp1 = head1,
    temp2 = head2;
  while (temp1 != null && temp2 != null) {
    if (temp1.val <= temp2.val) {
      temp.next = temp1;
      temp1 = temp1.next;
    } else {
      temp.next = temp2;
      temp2 = temp2.next;
    }
    temp = temp.next;
  }
  if (temp1 != null) {
    temp.next = temp1;
  } else if (temp2 != null) {
    temp.next = temp2;
  }
  return dummyHead.next;
};
var tosortList = function (head, tail) {
  if (head == null) {
    return head;
  }
  if (head.next == tail) {
    head.next = null;
    return head;
  }
  let slow = head,
    fast = head;
  while (fast != tail) {
    slow = slow.next;
    fast = fast.next;
    if (fast != tail) {
      fast = fast.next;
    }
  }
  let mid = slow;
  return marge(tosortList(head, mid), tosortList(mid, tail));
};
var sortList = function (head) {
  return tosortList(head, null);
};