leetcode-206-reverse-linked-list

Description

Reverse a singly linked list.

Example:

1 2	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?

####思路0

題意是將一個有序鏈錶反轉，首先使用迭代方法來遍歷列表。假設n是列表的長度，時間複雜度為$O(n)$

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
    }
    return prev;
    }
}

思路1

首先使用遞歸方法來遍歷列表。假設n是列表的長度，時間複雜度為$O(n)

public ListNode reverseList(ListNode head) {
    if (head == null || head.next == null) return head;
    ListNode p = reverseList(head.next);
    head.next.next = head;
    head.next = null;
    return p;
}