Java-反转单链表

public class ListNode {
private int val;
private ListNode next;

public ListNode(int value){
this.val = value;
this.next = null;
}

public int getVal() {
return val;
}
public void setVal(int val) {
this.val = val;
}
public ListNode getNext() {
return next;
}
public void setNext(ListNode next) {
this.next = next;
}
}

/**
* 遍历，双指针一前一后同时向尾部移动
* @param head 要反转的链表的头结点
* @return
*/
public static ListNode reverse(ListNode head){
if(head==null){
return head;
}
ListNode pre = head; 			//前一个指针
ListNode cur = head.getNext();  //后一个指针
ListNode next;					//临时对象，指针后移时要用到的
while(cur != null){
next = cur.getNext();
cur.setNext(pre);
pre = cur; //指针后移
cur = next;//指针后移
}

//一定记得把初始链表头结点的next置为null，因为反转后它将会是最后一个结点
head.setNext(null);
//把头结点设为反转后链表的第一个结点，然后返回就ok
head = pre;
return head;

}

/**
* 递归思想
* 在翻转当前节点之前，如果该节点有后续节点，先翻转其后续节点
* 如果该节点没有子节点，就把该节点的next指向其父节点，并把父节点的next置为null
* @param head
* @return
*/
public static ListNode reverse(ListNode head){
//当前结点为null，或者当前结点没有后续结点时，递归退出
if(head.getNext()==null || head==null){
return head;
}

//当前结点不为null并且有next时，递归去反转它的后续结点
ListNode tempNode = reverse(head.getNext());

//反转当前结点和它的子结点
head.getNext().setNext(head);
head.setNext(null);

return tempNode;
}

public static void main(String[] args){
ListNode node1 = new ListNode(1);
ListNode node2 = new ListNode(2);
ListNode node3 = new ListNode(3);
ListNode node4 = new ListNode(4);

node1.setNext(node2);
node2.setNext(node3);
node3.setNext(node4);

node1 = reverse(node1);

while(node1 != null){
System.out.print(node1.getVal()+" ");
node1 = node1.getNext();
}
}