leecode 143. reorder list

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
You may not modify the values in the list's nodes, only nodes itself may be changed.
Example 1:
Given 1->2->3->4, reorder it to 1->4->2->3.
Example 2:
Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        if (head == nullptr)
            return;
        ListNode* slow = head;
        ListNode* fast = head;
        while (slow->next && fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode* tmp = slow->next; //保存后半段list的头节点
        slow->next = nullptr;  //断开list
        
        //reverse list
        ListNode* prev = nullptr; 
        ListNode* node = tmp;
        ListNode* reverseHead = nullptr;
        while (node != nullptr) {
            ListNode* next = node->next;
            if (next == nullptr)
                reverseHead = node;
            node->next = prev;
            prev = node;
            node = next;
        }
        ListNode* p = reverseHead;
        ListNode* pNode = p;
        
        ListNode* oriNode = head;
        while (pNode != nullptr) {
            pNode = pNode->next;
            p->next = oriNode->next;
            oriNode->next = p;
            oriNode = p->next;
            p = pNode;
        }       
    }
};