48. 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 。
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：[]
static ListNode merge(ListNode l1, ListNode l2){ListNode head = new ListNode(-1);ListNode cur = head;while(l1 != null && l2 != null){if(l1.val < l2.val){cur.next = l1;l1 = l1.next;}else {cur.next = l2;l2 = l2.next;}cur =cur.next;}if(l1!= null){cur.next = l1;}if(l2 != null){cur.next = l2;}return head.next;}static ListNode sortList02(ListNode head, ListNode tail){if(head == null){return head;}if(head.next == tail){head.next = null;return head;}ListNode fast = head;ListNode slow = head;while(fast != null && fast != tail){fast = fast.next;slow = slow.next;if(fast != tail){fast = fast.next;}}ListNode mid = slow;ListNode l1 = sortList02(head, mid);ListNode l2 = sortList02(mid, tail);ListNode sorted = merge(l1,l2);return sorted;}static ListNode sort(ListNode head){ListNode res = sortList02(head, null);return res;}寻找链表的中点
ListNode fast = head;
ListNode slow = head;
while(fast != null && fast != tail){
fast = fast.next;
slow = slow.next;
if(fast != tail){
fast = fast.next;
}
}
链表插入排序
static ListNode sortList(ListNode head) {ListNode cur = head;ListNode newHead = new ListNode(-1);while (cur != null) {ListNode nextNode = cur.next;if (newHead.next == null) {newHead.next = cur;cur.next = null;} else {ListNode p = newHead.next;ListNode pre = newHead;while (p != null && p.val < cur.val) {pre = p;p = p.next;}if (p != null) {cur.next = p;pre.next = cur;}else{pre.next = cur;cur.next = null;}}cur = nextNode;}return newHead.next;}138. 随机链表的复制
用一个由 n 个节点组成的链表来表示输入/输出中的链表。每个节点用一个 [val, random_index] 表示：
val：一个表示 Node.val 的整数。
random_index：随机指针指向的节点索引（范围从 0 到 n-1）；如果不指向任何节点，则为 null 。
你的代码 只 接受原链表的头节点 head 作为传入参数。
示例 1：
输入：head = [[7,null],[13,0],[11,4],[10,2],[1,0]] 输出：[[7,null],[13,0],[11,4],[10,2],[1,0]]
示例 2：
输入：head = [[1,1],[2,1]] 输出：[[1,1],[2,1]]
示例 3：
输入：head = [[3,null],[3,0],[3,null]] 输出：[[3,null],[3,0],[3,null]]
public Node copyRandomList(Node head) {if(head == null){return null;}if(!cached.containsKey(head)){Node node = new Node(head.val);cached.put(head,node);node.next = copyRandomList(head.next);node.random = copyRandomList(head.random);}return cached.get(head);}这道题挺唬人的，但是只要记住管他随机链表，只要复制节点数据就行
