LeetCode 21 Merge Two Sorted Lists

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
    if(l1==NULL)
        return l2;
    if(l2==NULL)
        return l1;

    struct ListNode* head;    
    if(l1->val > l2->val)
    {
        head = l2;
        l2=l2->next;
    }
    else
    {
        head = l1;
        l1=l1->next;
    }
    
    struct ListNode* l3=head;
    while(l1 && l2)
    {
        if(l1->val > l2->val)
        {
            l3->next = l2;
            l2 = l2->next;
        }
        else
        {
            l3->next = l1;
            l1 = l1->next;
        }
        l3 = l3->next;
    }
    
    if(l1)
        l3->next=l1;
        
    if(l2)
        l3->next=l2;

    return head;
    
}