反转链表

leetcode :206.

需求：

反转链表

原链表：

graph LR A --> B --> C -->D -->null

反转后：

graph RL D -->C --> B --> A -->null

graph LR D-->C-->B-->A-->null

双指针法：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode ReverseList(ListNode head) {
        //比如有 pre,1，2，3 在1 之前
        ListNode  pre = null;
        var current = head;

        while (current != null)
        {
            //先记录下下一个节点，不然一会找不到了
            var temp = current.next;
            //开始反转
            current.next = pre;
            pre = current;
            current = temp;
        }

        //1-->2-->3-->null
        //当current为null时 pre 刚好为最后一个节点3
        return pre;
    }
}

递归写法：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode ReverseList(ListNode head) {
       return reverse(head,null);
    }

    ListNode reverse(ListNode current,ListNode pre)
    {
        if(current == null)
        return null;
        var temp = current.next;
        current.next = pre;
        var result= reverse(temp,current) ;
        return result ==null? current :result;
    }
}