21. Merge Two Sorted Lists

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

Problem

Merge two sorted linked lists and return it as a new sorted list. The new list should be made by splicing together the nodes of the first two lists.

Example:

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

Solution

1. With splicing

/**
 * 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* l1, ListNode* l2) {
        ListNode *const head = new ListNode();
        ListNode *current = head;
        while (l1 != nullptr && l2 != nullptr) {
            if (l1->val < l2->val) {
                current->next = l1;
                l1 = l1->next;
            } else {
                current->next = l2;
                l2 = l2->next;
            }
            current = current->next;
        }
        if (l1 != nullptr) current->next = l1;
        if (l2 != nullptr) current->next = l2;
        return head->next;
    }
};

2. Without splicing

/**
 * 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* l1, ListNode* l2) {
        ListNode *const head = new ListNode();
        ListNode *current = head;
        while (l1 != nullptr && l2 != nullptr) {
            if (l1->val < l2->val) {
                current->next = new ListNode(l1->val);
                l1 = l1->next;
            } else {
                current->next = new ListNode(l2->val);
                l2 = l2->next;
            }
            current = current->next;
        }
        while (l1 != nullptr) {
            current->next = new ListNode(l1->val);
            l1 = l1->next;
            current = current->next;
        }
        while (l2 != nullptr) {
            current->next = new ListNode(l2->val);
            l2 = l2->next;
            current = current->next;
        }
        return head->next;
    }
};

• #linkedlist