[One question per day] Merging two ordered linked lists

https://leetcode-cn.com/problems/merge-two-sorted-lists/

Combine the two ascending linked lists into a new ascending linked list and return. The new linked list is composed of all the nodes of the given two linked lists.

Examples:

Input: 1-> 2-> 4, 1-> 3-> 4
Output: 1-> 1-> 2-> 3-> 4-> 4

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Solution 1

Recursively. 8ms, 7.1mb

/**
 * 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 == nullptr) {
            return l2;
        }
        if (l2 == nullptr) {
            return l1;
        }

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

Solution 2

Iterate. 20 ms 7 MB

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* res = new ListNode(-1);
        ListNode* p = res;
        while (l1 != nullptr && l2 != nullptr) {
            if (l1->val <= l2->val) {
                p->next = l1;
                l1 = l1->next;
            } else {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        p->next = l1 == nullptr ? l2 : l1;

        return res->next;
    }
};

---

EOF