class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *l3;   //最终的链表头
        
        if (!l1) {
            return l2;
        } else if (!l2) {
            return l1;
        }
        
        ListNode *pTemp1 = l1;
        ListNode *pTemp2 = l2;
        ListNode *pTemp3;   //指向结果链表的最后一个节点
        
        //头节点特殊处理
        if (pTemp1->val < pTemp2->val) {
            l3 = pTemp1; pTemp3 = l3;
            pTemp1 = pTemp1->next;
        } else if (pTemp1->val > pTemp2->val) {
            l3 = pTemp2; pTemp3 = l3;
            pTemp2 = pTemp2->next;
        } else {
            l3 = pTemp1; pTemp1 = pTemp1->next; pTemp3 = l3;
            pTemp3->next = pTemp2; pTemp3 = pTemp2; pTemp2 = pTemp2->next;
        }
        
        //一个一个节点进行扫描
        while (pTemp1 && pTemp2) {
            if (pTemp1->val < pTemp2->val) {
                pTemp3->next = pTemp1; pTemp3 = pTemp1; pTemp1 = pTemp1->next;
            } else if (pTemp1->val > pTemp2->val) {
                pTemp3->next = pTemp2; pTemp3 = pTemp2; pTemp2 = pTemp2->next;
            } else {
                pTemp3->next = pTemp1;  pTemp3 = pTemp1; pTemp1 = pTemp1->next;
                pTemp3->next = pTemp2;  pTemp3 = pTemp2; pTemp2 = pTemp2->next;
            }
        }
        
        //剩余节点
        if (pTemp1) {
            pTemp3->next = pTemp1;
        }else if (pTemp2) {
            pTemp3->next = pTemp2;
        }
        
        return l3;
    }
};

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;
        }
    }
};