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/*
  最后也可以写为：
    cur.next = l1 == null ? l2 : l1;
    return head.next;
 */

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode cur = head;
        
        while(l1 != null && l2 != null) {
            if(l1.val > l2.val) {
                cur.next = l2;
                cur = cur.next;
                l2 = l2.next;
            } else {
                cur.next = l1;
                cur = cur.next;
                l1 = l1.next;        
            }
        }
        
        while(l1 != null) {
            cur.next = l1;
            cur = cur.next;
            l1 = l1.next;  
        }

        while(l2 != null) {
            cur.next = l2;
            cur = cur.next;
            l2 = l2.next;  
        }
        
        return head.next;
        // cur.next = l1 == null ? l2 : l1;
        // return head.next;
    }
}

/*
⭐️
*/

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;
        if (l1.val < l2.val) {
            l1.next = mergeTwoLists(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists(l1, l2.next);
            return l2;
        }
    }
}