reorderList.cpp

class Solution {
public:
    void reorderList(ListNode* head) {
        if (!head || !head->next) {
            return;
        }
        
        // Find the middle node of the list
        ListNode *slow = head, *fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        
        // Reverse the second half of the list
        ListNode *prev = nullptr, *curr = slow->next;
        slow->next = nullptr;
        while (curr) {
            ListNode *next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        
        // Merge the two halves of the list
        ListNode *p1 = head, *p2 = prev;
        while (p1 && p2) {
            ListNode *next1 = p1->next, *next2 = p2->next;
            p1->next = p2;
            p2->next = next1;
            p1 = next1;
            p2 = next2;
        }
    }
};