复制带随机指针的链表

leetcode 138

    //hashmap回溯
    HashMap <Node,Node> map = new HashMap<>();
    public Node copyRandomList(Node head) {
        if(head==null) return null;
        if(map.containsKey(head)) return map.get(head);
        Node node  = new Node(head.val);
        map.put(head,node);
        node.next = copyRandomList(head.next);
        node.random = copyRandomList(head.random);
        return node;
    }
    //hashmap迭代
    HashMap <Node,Node> map = new HashMap<>();
    public Node clone(Node head){
        if(head==null) return null;
        if(map.containsKey(head)) return map.get(head);
        else{
            Node node  = new Node(head.val);
            map.put(head,node);
            return node;
        }
    }
    public Node copyRandomList(Node head) {
        if(head==null) return null;
        Node node  = new Node(head.val);
        Node old = head;
        map.put(old,node);
        while(old!=null){
            node.next=clone(old.next);
            node.random=clone(old.random);
            node = node.next;
            old = old.next;
        }
        return map.get(head);
    }
        //空间O(1)的方法
    public Node copyRandomList(Node head) {
        //判空必须得写，否则报错
        if(head==null) return null;
        Node h = head;
        Node ptr;
        //将新旧结点相隔连接在一起
        while(h!=null){
            ptr = new Node(h.val);
            ptr.next = h.next;
            h.next = ptr;
            h = h.next.next;
        }
        //更新新结点的random
        h = head;
        while(h!=null){
            h.next.random = (h.random == null)?null:h.random.next;
            h = h.next.next;
        }
        //将新结点next相连接
        h = head;
        ptr = head.next;
        Node p = ptr;
        while(h!=null){
            h.next = ptr.next;
            ptr.next = (ptr.next == null)?null:ptr.next.next;
            h = h.next;
            ptr = ptr.next;
        }
        return p;
    }