java实现链表反转

/**
 * Created By --- on 2021/8/12
 * 以下代码可以直接粘贴进编译器输出
 */
public class ReverseList {
 
  public static void main(String[] args) {
    ListNode head = new ListNode(3, new ListNode(5, new ListNode(8, new ListNode(9))));
    System.out.println("初始链表：" + head);
 
    ListNode newList = reverseList(head);
    System.out.println("使用迭代法反转链表：" + newList);
 
    ListNode newList2 = reverseList2(null, newList);
    System.out.println("使用递归法反转链表：" + newList2);
  }
 
  /**
   * 迭代法
   */
  public static ListNode reverseList(ListNode head) {
    ListNode pre = null;
    ListNode cur = head;
    ListNode tmp;
    while (cur != null) {
      tmp = cur.next;
      cur.next = pre;
      pre = cur;
      cur = tmp;
    }
    return pre;
  }
 
  /**
   * 递归法
   */
  public static ListNode reverseList2(ListNode pre, ListNode cur) {
    if (cur == null) {
      return pre;
    }
 
    ListNode tmp = cur.next;
    cur.next = pre;
    pre = cur;
    cur = tmp;
    return reverseList2(pre, cur);
  }
 
 
}
 
 
/**
 * singly-linked list
 */
class ListNode {
 
  int val;
  ListNode next;
 
  ListNode() {
  }
 
  ListNode(int val) {
    this.val = val;
  }
 
  ListNode(int val, ListNode next) {
    this.val = val;
    this.next = next;
  }
 
  @Override
  public String toString() {
    StringBuilder sb = new StringBuilder(String.valueOf(val));
    ListNode next = this.next;
    while (next != null) {
      sb.append(next.val);
      next = next.next;
    }
    return sb.toString();
  }
}

import java.util.Stack;
 
public class ReverseLinkedList {
 
    public static void main(String[] args) {
        ReverseLinkedList reverseLinkedList = new ReverseLinkedList();
        reverseLinkedList.test();
    }
 
    public void test() {
        Node node1 = new Node(1);
        Node node2 = new Node(2);
        Node node3 = new Node(3);
        node1.setNext(node2);
        node2.setNext(node3);
        //方法需要替换
        node1 = reverseByPointer(node1);
        while (node1 != null) {
            System.out.println(node1.val);
            node1 = node1.getNext();
        }
    }
 
    //栈实现
    private Node reverseByStack(Node head) {
        if (head == null || head.getNext() == null) {
            return head;
        }
        Stack<Node> stack = new Stack<>();
        while (head != null) {
            stack.push(head);
            head = head.getNext();
        }
        head = stack.pop();
        Node tmp = head;
        while (!stack.empty()) {
            Node node = stack.pop();
            node.setNext(null);
            tmp.setNext(node);
            tmp = node;
        }
        return head;
    }
 
    //递归实现
    private Node reverseByRecursion(Node head) {
        if (head == null || head.getNext() == null) {
            return head;
        }
        //递归获取当前节点的后一个节点
        Node tmp = reverseByRecursion(head.getNext());
        Node node = head.getNext();
        head.setNext(null);
        node.setNext(head);
        return tmp;
    }
 
    //指针实现
    private Node reverseByPointer(Node head) {
        if (head == null || head.getNext() == null) {
            return head;
        }
        //pre指针指向前一个节点，初始第一个节点的前节点为空
        Node pre = null;
        //tmp指针指向当前节点
        Node tmp = null;
        while (head != null) {
            //tmp指针指向head头指针节点
            tmp = head;
            //head头指针向后遍历
            head = head.getNext();
            //反转，设置当前节点的下一个节点为前一个节点
            tmp.setNext(pre);
            //pre指针向后移动，指向当前节点
            pre = tmp;
        }
        return tmp;
    }
 
    private class Node {
        private int val;
 
        private Node next;
 
        public Node(int val) {
            this.val = val;
        }
 
        public Node getNext() {
            return next;
        }
 
        public void setNext(Node next) {
            this.next = next;
        }
    }
}