143. Reorder List(List)

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-2→…

You must do this in-place without altering the nodes‘ values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

class Solution {
public:
    void reorderList(ListNode *head) {
        if (!head|| !head->next)
        return;  

        // partition the list into 2 sublists of equal length
        ListNode *slowNode = head, *fastNode = head;
        while (fastNode->next) {
            fastNode = fastNode->next;
            if (fastNode->next) {
                fastNode = fastNode->next;
            } else {
                break;
            }
            slowNode = slowNode->next;
        }
        // 2 sublist heads
        ListNode *head1 = head, *head2 = slowNode->next;
        // detach the two sublists
        slowNode->next = NULL;  

        // reverse the second sublist
        ListNode *cur = head2, *post = cur->next;
        cur->next = NULL;
        while (post) {
            ListNode* temp = post->next;
            post->next = cur;
            cur = post;
            post = temp;
        }
        head2 = cur; // the new head of the reversed sublist  

        // merge the 2 sublists as required
        ListNode* p = head1, *q = head2;
        while (q ) {
            ListNode *temp1 = p->next;
            ListNode *temp2 = q->next;
            p->next = q;
            q->next = temp1;
            p = temp1;
            q = temp2;
        }
    }
};