二路归并
/**
* 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) {
ListNode* l3,*l4;
if(l1==NULL||l2==NULL)
return l1==NULL?l2:l1;
if(l1->val>l2->val)
{
l3=l2;
l2=l2->next;
}else{
l3=l1;
l1=l1->next;
}
l4=l3;
while(l1!=NULL&&l2!=NULL)
{
if(l1->val>l2->val)
{
l4->next=l2;
l4=l4->next;
l2=l2->next;
}else{
l4->next=l1;
l4=l4->next;
l1=l1->next;
}
}
if(l1==NULL)
{
l4->next=l2;
}else{
l4->next=l1;
}
return l3;
}
};