牛客网剑指offer刷题Java版-15反转链表

题目描述
输入一个链表，反转链表后，输出新链表的表头。

栈法：

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
import java.util.Stack;
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null)
            return null;
        Stack <ListNode> stack=new Stack();
        while(head.next!=null){
            stack.push(head);
            head=head.next;
        }
        ListNode temp=head;
        while(!stack.isEmpty()){
            temp.next=stack.pop();
            temp=temp.next;
        }
        temp.next=null;
        return head;
    }
}

指针法：

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
import java.util.Stack;
public class Solution {
    public ListNode ReverseList(ListNode head) {
        ListNode pre=null;
        ListNode next=null;
        while(head!=null){
            next=head.next;
            head.next=pre;
            pre=head;
            head=next;
        }
        return pre;
    }
}