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){
    struct ListNode* head = NULL;
    if(l1 && l2){
        if(l1->val <= l2->val){
            head = l1;
            l1 = l1->next;
        }else{
            head = l2;
            l2 = l2->next;
        }
        struct ListNode* p = head;
        for(p=head;p->next;p=p->next){
            if(l1->val <= l2->val){
                p->next = l1;
                l1 = l1->next;
            }else{
                p->next = l2;
                l2 = l2->next;
            }
        }
        while(p){
            if(l1 == NULL){
                p->next = l2;
                if(l2 != NULL){
                    l2 = l2->next;
                }
                p = p->next;
            }else{
                p->next = l1;
                if(l1 != NULL){
                    l1 = l1->next;
                }
                p = p->next;
            }
        }
    }else if(l1 || l2){
        if(l1 == NULL){
            head = l2;
        }else{
            head = l1;
        }
    }
    

    return head;
}