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

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

思路:

代码：

    void reorderList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        stack<ListNode *> s;
        vector<ListNode *> v;
        if(head == NULL)
            return;
        ListNode *tmp = head;
        int n = 0;
        while(tmp){
            n++;
            s.push(tmp);
            v.push_back(tmp);
            tmp = tmp->next;
        }
        int i;
        ListNode *n1, *n2;
        for(i = 0; i < n/2; i++){
            n1 = v[i];
            n2 = s.top();
            s.pop();
            n2->next = n1->next;
            n1->next = n2;
        }
        v[n/2]->next = NULL;
    }