Merge Two Sorted List

ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
    
    if(NULL == l1) return l2;
    if(NULL == l2) return l1;
    
    ListNode* head=NULL;    // head of the list to return
    
    // find first element (can use dummy node to put this part inside of the loop)
    if(l1->val < l2->val)       { head = l1; l1 = l1->next; }
    else                        { head = l2; l2 = l2->next; }
    
    ListNode* p = head;     // pointer to form new list
    
    // I use && to remove extra IF from the loop
    while(l1 && l2){
        if(l1->val < l2->val)   { p->next = l1; l1 = l1->next; }
        else                    { p->next = l2; l2 = l2->next; }
        p=p->next;
    }
    
    // add the rest of the tail, done!
    if(l1)  p->next=l1;
    else    p->next=l2;
    
    return head;
}
leetcode上的非递归解法

注意：要返回的临时列表要有一个指针充当表头，添加元素的操作为先让.next指针指向要添加的元素，再使p指针指向这个元素。

边界条件的判别：空链表