合并两个有序链表

/**
 * 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) {
        if(l1 == nullptr && l2 == nullptr){
            return l1;
        }
        if(l1 == nullptr && l2 != nullptr){
            return l2;
        }
        if(l1 != nullptr && l2 == nullptr){
            return l1;
        }
        
        ListNode *head = nullptr;
        ListNode *tail = nullptr;
        if(l1->val <= l2->val){
            head = l1;
            tail = l1;
            l1 = l1->next;
        }
        else{
            head = l2;
            tail = l2;
            l2 = l2->next;
        }
        while(!(l1 == nullptr || l2 == nullptr)){
            if(l1->val <= l2->val){
                tail->next = l1;
                l1 = l1->next;
            }
            else{
                tail->next = l2;
                l2 = l2->next;
            }
            tail = tail->next;
        }
        if(l1 == nullptr){
            tail->next = l2;
        }
        if(l2 == nullptr){
            tail->next = l1;
        }
        
        return head;
    }
};