迭代

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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 ;            //剩余部分的加入
    return dummy.next;
    }
};

递归

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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);
        else                            l2->next = mergeTwoLists(l1,l2->next);

        return l1->val < l2->val ? l1 : l2 ;      
    }
};