143. Reorder List

[jason89521]

Practice Dates

• 2024/05/13

Description

Link: https://leetcode.com/problems/reorder-list/description/

Solution

Reverse the second half list, then merge the first half list and the reversed list.

/**
 * 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) {}
 * };
 */
ListNode* reverseList(ListNode* head) {
    ListNode* cur = head, *prev = nullptr;
    while (cur != nullptr) {
        ListNode* next = cur->next;
        cur->next = prev;
        prev = cur;
        cur = next;
    }

    return prev;
}

class Solution {
public:
    void reorderList(ListNode* head) {
        ListNode* firstTail = head, *fast = head;
        // find the tail of the second half list
        while (fast != nullptr) {
            fast = fast->next;
            if (fast == nullptr) {
                break;
            }
            fast = fast->next;
            if (fast == nullptr) {
                break;
            }
            firstTail = firstTail->next;
        }
        ListNode* secondHead = firstTail->next;
        secondHead = reverseList(secondHead);
        firstTail->next = nullptr;

        ListNode* temp = secondHead;

        while (head != nullptr && secondHead != nullptr) {
            ListNode* firstNext = head->next;
            ListNode* secondNext = secondHead->next;
            head->next = secondHead;
            secondHead->next = firstNext;
            head = firstNext;
            secondHead = secondNext;
        }
    }
};

Performance

Time complexity: