NC33 #合并有序链表#

非递归方法：

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;
        ListNode* pHead = nullptr;
        if(l1->val < l2->val) 
            pHead = l1, l1 = l1->next;
        else pHead = l2, l2 = l2->next;
        ListNode* pTail = pHead;
        while(l1 != nullptr && l2 != nullptr)
        {
            if(l1->val < l2->val)
                pTail->next = l1, l1 = l1->next;
            else
                pTail->next = l2, l2 = l2->next;
            pTail = pTail->next;
        }
        if(l1 == nullptr) pTail->next = l2;
        else pTail->next = l1;
        return pHead;
    }
};

递归方法：

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(!l1) return l2;
        if(!l2) return l1;
        if(l1->val<=l2->val)
        {
            l1->next = mergeTwoLists(l1->next,l2);
            return l1;
        }
        else
        {
            l2->next = mergeTwoLists(l1,l2->next);
            return l2;
        }
    }
};