[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;

    }

};