Merge two sorted Lists

merge_two_sorted_list.cpp

``` 
class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(l1==NULL) return l2;
        if(l2==NULL) return l1;
        ListNode *p1=l1;
        ListNode *p2=l2;
        ListNode *head=NULL;
        ListNode *cur=NULL;
        while(p1&&p2){
            if(head==NULL){
                if(p1->val<=p2->val){
                    head=cur=p1;
                    p1=p1->next;
                }else{
                    head=cur=p2;
                    p2=p2->next;
                }
            }else{
                if(p1->val<=p2->val){
                    cur->next=p1;
                    cur=p1;
                    p1=p1->next;
                }else{
                    cur->next=p2;
                    cur=p2;
                    p2=p2->next;
                }
            }
        }
        if(p1) cur->next=p1;
        if(p2) cur->next=p2;
        return head;
    }
};

```