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;  
        }  
    }  
};