206. Reverse Linked List

Reverse a singly linked list.

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Hint:

A linked list can be reversed either iteratively or recursively. Could you implement both?

这道题可以看看这篇文章的指导：http://algorithms.tutorialhorizon.com/reverse-a-linked-list/，会对什么是Reverse Linked List有个清晰的认识。只需要把currentNode的next改成preNode就可以啦。附上迭代和递归两种解法的代码：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode reverseList(ListNode head) {
        /*ListNode nextNode = null;
        ListNode preNode = null;
        ListNode currNode = head;
        while (currNode != null) {
            nextNode = currNode.next;
            currNode.next = preNode;
            preNode = currNode;
            currNode = nextNode;
        }
        return preNode;*/
        return reverseSignalList(null, head, null);
    }
    public ListNode reverseSignalList(ListNode preNode, ListNode currNode, ListNode nextNode) {
        if (currNode == null) {
            return preNode;
        }
        nextNode = currNode.next;
        currNode.next = preNode;
        preNode = currNode;
        currNode = nextNode;
        return reverseSignalList(preNode, currNode, nextNode);
    }
}