牛客网初级班------练习7之反转单向和双向链表

反转单向和双向链表 【题目】

分别实现反转单向链表和反转双向链表的函数。 【要求】 如果链表长度为N，时间复杂度要求为O(N)，额外空间 复杂度要求为O(1)

具体代码如下：

public static class Node {
   public int value;
   public Node next;

   public Node(int data) {
      this.value = data;
   }
}

public static Node reverseList(Node head) {
   Node pre = null;
   Node next = null;
   while (head != null) {
      next = head.next;
      head.next = pre;
      pre = head;
      head = next;
   }
   return pre;
}

public static class DoubleNode {
   public int value;
   public DoubleNode last;
   public DoubleNode next;

   public DoubleNode(int data) {
      this.value = data;
   }
}

public static DoubleNode reverseList(DoubleNode head) {
   DoubleNode pre = null;
   DoubleNode next = null;
   while (head != null) {
      next = head.next;
      head.next = pre;
      head.last = next;
      pre = head;
      head = next;
   }
   return pre;
}

public static void printLinkedList(Node head) {
   System.out.print("Linked List: ");
   while (head != null) {
      System.out.print(head.value + " ");
      head = head.next;
   }
   System.out.println();
}

public static void printDoubleLinkedList(DoubleNode head) {
   System.out.print("Double Linked List: ");
   DoubleNode end = null;
   while (head != null) {
      System.out.print(head.value + " ");
      end = head;
      head = head.next;
   }
   System.out.print("| ");
   while (end != null) {
      System.out.print(end.value + " ");
      end = end.last;
   }
   System.out.println();
}

public static void main(String[] args) {
   Node head1 = new Node(1);
   head1.next = new Node(2);
   head1.next.next = new Node(3);
   printLinkedList(head1);
   head1 = reverseList(head1);
   printLinkedList(head1);

   DoubleNode head2 = new DoubleNode(1);
   head2.next = new DoubleNode(2);
   head2.next.last = head2;
   head2.next.next = new DoubleNode(3);
   head2.next.next.last = head2.next;
   head2.next.next.next = new DoubleNode(4);
   head2.next.next.next.last = head2.next.next;
   printDoubleLinkedList(head2);
   printDoubleLinkedList(reverseList(head2));

}