Reorder List.cpp

/**
 * 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 || !head->next)return;
        ListNode* fast = head, *slow = head;
        //partition
        while(fast->next && fast->next->next)
        {
            fast = fast->next->next;
            slow = slow->next;
        }
        auto head2 = slow->next;
        slow->next = nullptr;
        //reverse second
        head2 = reverse(head2);
        //merge second list into first list
        auto curr1 = head, curr2 = head2;
        while(curr2)
        {
            auto tmp = curr2->next;
            curr2->next = curr1->next;
            curr1->next = curr2;
            curr1 = curr1->next->next;
            curr2 = tmp;
        }
    }

private:
    ListNode* reverse(ListNode* head)
    {
        ListNode* prev = head, *curr = head->next;
        prev->next = nullptr;
        while(curr)
        {
            auto tmp = curr->next;
            curr->next = prev;
            prev = curr;
            curr = tmp;
        }
        return prev;
    }
};