# 206     Reverse Linked List

## 206. [Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/description/)

## 1. Question

Reverse a singly linked list.

**Hint:**

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

## 2. Implementation

(1) Iterative

```java
/**
 * 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 preNode = null, curNode = head, nextNode = null;

        while (curNode != null) {
            nextNode = curNode.next;
            curNode.next = preNode;
            preNode = curNode;
            curNode = nextNode;
        }
        return preNode;
    }
}
```

## 3. Time & Space Complexity

Iterative: 时间复杂度O(n), 空间复杂度O(1)