Leetcode: Reverse Linked List

Example:

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?

基本迭代

Time: O(N), Space: O(1)

class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) return null;
        ListNode p1 = head;
        ListNode p2 = head.next;
        while (p2 != null) {
            ListNode next = p2.next;
            p2.next = p1;
            p1 = p2;
            p2 = next;
        }
        head.next = null;
        return p1;
    }
}

基本递归

Time: O(N), Space: O(N)递归栈大小

class Solution {
    public ListNode reverseList(ListNode head) {
        if (head == null) return null;
        List<ListNode> res = new ArrayList<>();
        res.add(null);
        reverse(head, res);
        return res.get(0);
    }
    
    public ListNode reverse(ListNode node, List<ListNode> res) {
        if (node.next == null) {
            res.set(0, node);
            return node;
        }
        ListNode afterReverse = reverse(node.next, res);
        afterReverse.next = node;
        node.next = null;
        return node;
    }
}