206 Reverse Linked List

206. Reverse Linked List

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

/**
 * 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)