Day01

迭代+递归

package LeetCode;

public class L1_ReverseLinkedlist {

    static class ListNode {
        int val;
        ListNode next;

        public ListNode(int val, ListNode next) {
            this.val = val;
            this.next = next;
        }
    }

//迭代
    public static ListNode iterats(ListNode head) {
        ListNode prev = null;
        ListNode next = null;
        ListNode curr = head;
        while ( curr != null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }
//递归
    public static ListNode Recursion(ListNode head){
        if (head == null || head.next == null){
            return head;
        }
        ListNode new_head = Recursion(head.next);
        head.next.next = head;
        head.next = null;
        return new_head;
    }

    public static void main(String[] args) {
        ListNode node5 = new ListNode(5, null);
        ListNode node4 = new ListNode(4, node5);
        ListNode node3 = new ListNode(3, node4);
        ListNode node2 = new ListNode(2, node3);
        ListNode node1 = new ListNode(1, node2);

        iterats(node1);
        ListNode prev = Recursion(node1);
        System.out.println(prev);
    }
}