Interview Classic 150 Problem P21: Merge Two Sorted Linked Lists

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Timeline

Timeline

2025-11-19

init

linked list

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Problem:

P21 Merge Two Sorted Linked Lists

https://leetcode.cn/problems/merge-two-sorted-lists/description/?envType=study-plan-v2&envId=top-interview-150

You can first create a head node to facilitate tail insertion, and then release it at the end.

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)
	{
		// Create a head node
		ListNode *list3_head = new ListNode;
		ListNode *p = list3_head;

		while (list1 != nullptr || list2 != nullptr) {
			if (list1 == nullptr && list2 != nullptr) {
				p->next = list2;
				list2 = list2->next;
				p = p->next;
				p->next = nullptr;
			} else if (list1 != nullptr && list2 == nullptr) {
				p->next = list1;
				list1 = list1->next;
				p = p->next;
				p->next = nullptr;
			} else if (list1 != nullptr && list2 != nullptr) {
				if (list1->val <= list2->val) {
					p->next = list1;
					list1 = list1->next;
					p = p->next;
					p->next = nullptr;
				} else {
					p->next = list2;
					list2 = list2->next;
					p = p->next;
					p->next = nullptr;
				}
			}
		}
		p = list3_head;
		list3_head = list3_head->next;
		delete p;
		return list3_head;
	}
};

leetcode hot100 rewrite

/**
 * 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) {}
 * };
 */
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)
        {
                ListNode *virtual_head = new ListNode;
                ListNode *tail = virtual_head, *p;

                while (list1 != nullptr || list2 != nullptr) {
                        if (list1 && list2)
                                tail->next = list1->val < list2->val ? list1 : list2;
                        else
                                tail->next = (list1 == nullptr) ? list2 : list1;

                        if (tail->next == list1)
                                list1 = list1->next;
                        if (tail->next == list2)
                                list2 = list2->next;

                        tail = tail->next;
                }

                tail = virtual_head->next;
                delete virtual_head;
                return tail;
        }
};