simple 合併兩個有序鏈表

題目

將兩個升序鏈表合併爲一個新的升序鏈表並返回。新鏈表是通過拼接給定的兩個鏈表的所有節點組成的。

示例：
輸入：1->2->4, 1->3->4
輸出：1->1->2->3->4->4

//cpp
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* n1, ListNode* n2) {
    ListNode  dummy (0) ;      
    ListNode* start= &dummy ;  

    while(n1 && n2)
    {
        if(n1->val <= n2->val)   
        {
            start->next = n1;
            n1 = n1->next;
        }
        else                    
        {
            start->next = n2;
            n2 = n2->next;
        }
        start = start->next;
    }
    start->next = n1 ? n1 : n2 ;            //剩餘部分的加入
    return dummy.next;
    }
};

#python
class Solution:
    def mergeTwoLists(self, l1, l2):
        if l1 is None:
            return l2
        elif l2 is None:
            return l1
        elif l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2