[LeetCode] Merge Two Sorted Lists

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.

/**
 * 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) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(l1 == NULL && l2 == NULL)
            return NULL;
        if(l1 == NULL)
            return l2;
        if(l2 == NULL)
            return l1;
        
        ListNode *root, *last, *t_l1 = l1, *t_l2 = l2;
        if(t_l1 -> val < t_l2 -> val)
        {
            root = t_l1;
            last = t_l1;
            t_l1 = t_l1 -> next;
        }
        else
        {
            root = t_l2;
            last = t_l2;
            t_l2 = t_l2 -> next;
        }
        
        while(true)
        {
            if(t_l1 == NULL)
            {
                if(t_l2 == NULL)
                {
                    break;   
                }
                else
                {
                    last -> next = t_l2;
                    break;
                }
            }
            else
            {
                if(t_l2 == NULL)
                {
                    last -> next = t_l1;
                    break;
                }
                else
                {
                    if(t_l1 -> val < t_l2 -> val)
                    {
                        last -> next = t_l1;
                        last = t_l1;
                        t_l1 = t_l1 -> next;
                    }
                    else
                    {
                        last -> next = t_l2;
                        last = t_l2;
                        t_l2 = t_l2 -> next;
                    }
                }
            }
        }
        
        return root;
    }
};