21. Merge Two Sorted Lists 기초 문제 의 두 가지 정련 해법

방법 1: 귀속

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

법 2: 재 귀적 인 정련 코드:

/**
 * 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) {
        ListNode dummy(1);
        ListNode* tail = &dummy;//  ，         ，    dummy   ，  new ，         
        while(l1 && l2){
            if(l1->val > l2->val){
                tail->next = l2;
                l2 = l2->next;
            }else{
                tail->next = l1;
                l1 = l1->next;
            }
            tail = tail->next;
        }
        //tail->next = l1 ? l1 : l2;
        l1 == NULL? tail->next = l2: tail->next = l1;
        return dummy.next;
    }
};