第 2 天 链表（简单）

class Solution {
    public Node copyRandomList(Node head) {
        if(head==null) return null;
        Node cur=head;
        //利用哈希表 map，key为 旧节点，value 为新节点：
        Map map=new HashMap();
        while(cur!=null){
            //顺序遍历原链表，将旧新节点键值对关系存储到哈希表map中。
            map.put(cur,new Node(cur.val));
            cur=cur.next;
        }
        cur=head;
        //再次从头遍历原链表，从 map 中通过旧节点取出新节点，构建新链表：
        while(cur!=null){
            map.get(cur).next=map.get(cur.next);
            map.get(cur).random=map.get(cur.random);
            cur=cur.next;
        }
        return map.get(head);
    }
}

class Solution {
    public int[] reversePrint(ListNode head) {
        LinkedList A=new LinkedList();
        ListNode p=head;
        int ans=0;
        while(p!=null){ 
            ans++;
            A.addFirst(p.val);
            p=p.next;
        }
        
        int b[]=new int[ans];
        int i=0;
        while(!A.isEmpty()){
            b[i++]=A.getFirst();
            A.removeFirst();
        }
        return b;
    }
}

class Solution {
    public ListNode reverseList(ListNode head) {
        LinkedList A=new LinkedList();
        ListNode p=head;
        while(p!=null){
            A.addFirst(p);
            p=p.next;
        }
        ListNode q = new ListNode(-1);
        p=q;
        while(!A.isEmpty()){
            p=p.next=A.getFirst();
            A.removeFirst();
        }
        p.next=null;
        return q.next;
    }
}

class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode pre = null, cur = head, next = null;
        while(cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}