leetcode刷题笔记 二百零六题 反转链表

leetcode刷题笔记 二百零六题 反转链表

源地址：206. 反转链表

问题描述：

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL
进阶:
你可以迭代或递归地反转链表。你能否用两种方法解决这道题？

//迭代 头插法
/**
 * Definition for singly-linked list.
 * class ListNode(_x: Int = 0, _next: ListNode = null) {
 *   var next: ListNode = _next
 *   var x: Int = _x
 * }
 */
object Solution {
    def reverseList(head: ListNode): ListNode = {
        var prev: ListNode = null
        var curr = head
        while (curr != null) {
            val temp = curr.next
            curr.next = prev
            prev = curr
            curr = temp
        }
        return prev
    }
}

//递归
// 。。。 -> k ->  k+1 <- k+2 。。。
//故 k.next.next = k
/**
 * Definition for singly-linked list.
 * class ListNode(_x: Int = 0, _next: ListNode = null) {
 *   var next: ListNode = _next
 *   var x: Int = _x
 * }
 */
object Solution {
    def reverseList(head: ListNode): ListNode = {
        if (head == null || head.next == null) return head
        val temp = reverseList(head.next)
        head.next.next = head
        head.next = null
        return temp
    }
}