Merge Two Sorted Lists

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/**
 * 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) {
        ListNode * head;
        if(l1 == NULL){
            return l2;
        }
        if(l2 == NULL){
            return l1;
        }
        
        if(l1 && l2){
            if(l1->val < l2->val){
                head = l1;
                l1 = l1->next;
            }
            else{
                head = l2;
                l2 = l2->next;
            }
        }
        ListNode *cur = head;
        while(l1 && l2){
            if(l1->val < l2->val){
                cur->next = l1;
                cur = cur->next;
                l1 = l1->next;
            }
            else{
                cur->next = l2;
                cur = cur->next;
                l2 = l2->next;
            }
        }
        if(l1){
            cur->next = l1;
        }
        if(l2){
            cur->next = l2;
        }
        return head;
    }
};