* Definition for singly-linked list.
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
void reorderList(ListNode* head) {
if(!head || !head->next || !head->next->next) return;
while( last->next && last->next->next)
stack<ListNode*> lastMid;
ListNode* tmp= lastMid.top();