# leetcode解題：(Easy) 206. Reverse Linked List

題目：[https://leetcode.com/problems/reverse-linked-list/](https://leetcode.com/problems/reverse-linked-list/)

描述：給定一個鏈結串列，回傳一個與之順序顛倒的鏈結串列

解題思路：這種問題畫圖模擬迭代過程應該是最簡單直覺也最好的方式，比較會卡住的部分是找出需要用多少個node來確保能順利迭代，以這題而言就需要3個(包含輸入給的head)

程式碼：

/**
 * 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 pre = null;
        ListNode curr = null;
        while(head != null) {
            pre = curr;
            curr = head;
            head = head.next;
            curr.next = pre;
        }

        return curr;
    }
}

時間複雜度：O(n)
空間複雜度：O(1)

###### tags: `leetcode` `easy` `linked list`