LC_21_MergeTwoSortedLists.cpp

/*
https://leetcode.com/problems/merge-two-sorted-lists/
21. Merge Two Sorted Lists

*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        if(!list1) 
            return list2;
        else if(!list2) 
            return list1;
            
        ListNode* head = new ListNode();
        ListNode* ptr = head;
        
        while(list1 && list2)
        {
            if(list1->val <= list2->val)
            {
                ptr->next = list1;
                list1 = list1->next;
            }
            else
            {
                ptr->next = list2;
                list2 = list2->next;
            }
            ptr = ptr->next;
        }
    
        if(list1) 
            ptr->next = list1;
        else
            ptr->next = list2;
        
//         while(list1)
//         {
//             ptr->next = list1;
//             list1 = list1->next;
//              ptr = ptr->next;
//         }
        
//         while(list2)
//         {
//             ptr->next = list2;
//             list2 = list2->next;
//              ptr = ptr->next;
//         }
        
        return head->next;
    }
};