/**
 * @Author：田宇寒.
 * @Date：Created in 9:56 2021/6/9
 * @Description：合并两个有序链表
 * @ModifiedBy：
 * @Version: 将两个升序链表合并为一个新的 升序 链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。
 */
public class code21 {

    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;
        }
    }

    public ListNode mergeTwoListsWithIteration(ListNode l1, ListNode l2) {
        /**
         * create by: 田宇寒
         * description: 迭代算法 时间复杂度O(m+n) 空间复杂度O(1)
         * create time: 10:19 2021/6/9
         * @Param: l1
         * @Param: l2
         * @return 'code21.ListNode'
         */
        ListNode head, tail;
        head = new ListNode(-1);
        tail = head;
        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                tail.next = l1;
                l1 = l1.next;
            } else {
                tail.next = l2;
                l2 = l2.next;
            }
            tail = tail.next;
        }
        tail.next = l1 == null? l2 : l1;
        return head.next;
    }

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        /**
         * create by: 田宇寒
         * description: 递归算法 时间复杂度O(m+n) 空间复杂度O(m+n)
         * create time: 10:24 2021/6/9
         * @Param: l1
         * @Param: l2
         * @return code21.ListNode
         */
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        } else if (l1.val <= l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}