LeetCode206-Reverse Linked List-Easy

Reverse a singly linked list.

Example:

Input: 1->2->3->4->5->NULL
Output: 5->4->3->2->1->NULL

Follow up:

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

两种解法：递归、迭代

递归法（Recursion）：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        
        if(head == null || head.next == null) {
            return head;
        }
        
        ListNode next = head.next;
        ListNode reverseHead = reverseList(next);
        next.next = head;
        head.next = null;
        return reverseHead;
    }
}

迭代法（iteration）

也叫头插法，每次取原链表中的第一个结点，插到新链表的头部。这样原链表第一个结点就会变成，新链表的最后一个结点。最终返回新链表头结点。

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