leetcode Merge Two Sorted Lists 통합 두 개의 정렬 체인 테이블

/**
 * 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 == NULL) return l2;
        if (l2 == NULL) return l1;
        if (l1->val < l2->val) {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
        else {
            l2->next = mergeTwoLists(l2->next, l2);
            return l2;
        }
    }
};

/**
 * 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 == NULL) return l2;
        if (l2 == NULL) return l1;
        ListNode *temphead = NULL;
        ListNode *l1pointer = l1;
        ListNode *l2pointer = l2;
        ListNode *temptail = NULL;
        // ， l1,l2 ， 
        if (l1->val < l2->val) {
            temphead = l1;
            l1pointer = l1pointer->next;
        }
        else {
            temphead = l2;
            l2pointer = l2pointer->next;
        }
        temptail = temphead;
        while (l1pointer != NULL && l2pointer != NULL) {
        // l1,l2 
            if (l1pointer->val < l2pointer->val){
                temptail->next = l1pointer;
                l1pointer = l1pointer->next;
            }
            else {
                temptail->next = l2pointer;
                l2pointer = l2pointer->next;                
            }
            temptail = temptail->next;
        }
        if (l1pointer == NULL)
            temptail->next = l2pointer;
        else
            temptail->next = l1pointer;
        return temphead;
    }
};