# C++ 2 solutions

1. Mutable, recursive

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;
}
``````

}

2. New list, iterative

ListNode * mergeTwoLists( ListNode * l1, ListNode * l2 )
{
if( !l1 ) return l2;
if( !l2 ) return l1;

``````     int min = l1->val;
if( min > l2->val)
{
min = l2->val;
l2 = l2->next;
}
else l1 = l1->next;

ListNode * out =new ListNode( min );
ListNode * next = out;

for( ;; )
{
if( l1 && l2 )
{
min = l1->val;
if( min > l2->val)
{
min = l2->val;
l2 = l2->next;
}
else l1 = l1->next;
ListNode * temp =new ListNode( min );

next->next = temp;
next=next->next;
}
else if( l1 )
{
ListNode * temp =new ListNode( l1->val );
next->next = temp;
next = next->next;
l1 = l1->next;
}
else if( l2 )
{
ListNode * temp =new ListNode( l2->val );
next->next = temp;
next = next->next;
l2 = l2->next;
}
else break;
}
return out;
}``````

• Actually, In solution 2, u don't need to new nodes, and can be impoved.

``````class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode *l3, *head;
head = l3;
if(l1 == NULL and l2 == NULL)return NULL;
if(l1 == NULL or l2 == NULL)return l1==NULL?l2:l1;
while(l1 and l2){
if(l1->val <= l2->val){
l3->next = l1;
l1 = l1->next;
l3 = l3->next;
}
else {
l3->next = l2;
l2 = l2->next;
l3 = l3->next;
}
}
if(l1){//just point to the left nodes if there have.
l3->next = l1;
}
if(l2){
l3->next = l2;
}
return head->next;
}
};``````

• Good answer.

• hello,after reading your codes,there may be some errors in your codes I think.It is theinitialization of l3 that may cause mistake,because at first ,your l3(pointer of ListNode) does not point to a real space of ListNode in memory,so when you begin running "l3->next=l1",it may cause mistake,l3 is just a pointer,but we can not find its real space in memory,so you may construct only one ListNode and make l3 be its point,(I am awfully sorry for my poor English).

• yeah, u are right. New a empty node to initialize l3 and head.顺带，我能看懂中文啊！哈哈哈哈哈哈哈哈

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