Merge Two Sorted Lists [Leetcode]

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Solution:pointer manipulation.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
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* c1 = (l1->val<=l2->val?l1:l2); //pointing to the head with the smaller val
        ListNode* c2 = (l1->val<=l2->val?l2:l1); //pointing to the head with the larger val
        while(c1->next!=NULL && c2!=NULL){
            if(c1->val<=c2->val && c2->val<=c1->next->val){
                //insert c2 behind of c1
                ListNode* tmp = c2->next;
                c2->next = c1->next;
                c1->next = c2;
                //advance both pointers
                c1 = c1->next;
                c2 = tmp;
            }
            else
                c1 = c1->next;
        }
        if(c1->next==NULL)
            c1->next = c2;
        return (l1->val<=l2->val?l1:l2);
    }
};