剑指 Offer 25. 合并两个排序的链表

class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dump = new ListNode(-1);
        ListNode p = dump;
        while(l1!=null && l2!=null){
            if(l1.val<=l2.val){
                p.next = new ListNode(l1.val);
                p = p.next;
                l1 = l1.next;
            }else{
                p.next = new ListNode(l2.val);
                p = p.next;
                l2 = l2.next;
            }
        }
        if (l1!=null){
            p.next = l1;
        }
        if(l2!=null){
            p.next = l2;
        }
        return dump.next;
    }
}

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方法二：

递归

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