leetcode 21: merge two sorted lists

 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* (struct ListNode* l1, struct ListNode* l2) {
    if (l1 == NULL){
        return l2;
    } 
    if (l2 == NULL){
        return l1;
    } 
    
    struct ListNode* head = NULL;
    
    // find the first element 
    if (l1->val < l2->val){
        head = l1;
        l1 = l1->next;
    }else{
        head = l2;
        l2 = l2->next;
    }
    
    struct ListNode* p = head;
    while(l1&&l2){
        if (l1->val < l2->val){
            p->next = l1;
            l1 = l1->next;
        }else{
            p->next = l2;
            l2 = l2->next;
        }
        p = p->next;
    }
    if(l1){
        p->next=l1;
    }else{
        p->next=l2;
    }    
    return head;
}