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struct ListNode {	int val;	ListNode *next;	ListNode(int x) : val(x), next(NULL) {}};class Solution {//this is a very simple implementation//one pointer and one pointer's pointerpublic:	ListNode * mergeTwoList(ListNode *l1, ListNode *l2) {		ListNode *h = NULL;		ListNode **p = &h;		while(l1 && l2) 		{			if(l1->val < l2->val)			{				*p = l1;				l1 = l1->next;			}			else			{				*p = l2;				l2 = l2->next;			}			p = &((*p)->next);		}		if(l1) *p = l1;		else *p = l2;		return h;	}};

second time

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        ListNode dummy(-1);        ListNode* prev = &dummy;        while(l1 != NULL && l2 != NULL)        {            ListNode* tmp;            if(l1->val < l2->val) tmp = l1, l1 = l1->next;            else tmp = l2, l2 = l2->next;            prev->next = tmp;            prev = prev->next;        }        if(l1 == NULL && l2 == NULL) prev->next = NULL;        if(l1 != NULL) prev->next = l1;        if(l2 != NULL) prev->next = l2;                return dummy.next;    }};

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