206_Reverse_Linked_List.java

/**
 * 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 {
    // https://leetcode.com/problems/reverse-linked-list/discuss/58125/In-place-iterative-and-recursive-Java-solution
    public ListNode reverseList(ListNode head) {
        // Iterative solution
        ListNode newHead = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
    
    // Recursive
    /*
    public ListNode reverseList(ListNode head) {
        return reverseListInt(head, null);
    }
    
    private ListNode reverseListInt(ListNode head, ListNode newHead) {
        if (head == null)
            return newHead;
        ListNode next = head.next;
        head.next = newHead;
        return reverseListInt(next, head);
    }*/
}