143

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        if(!head->next) return;
        ListNode *slow, *fast;
        slow = fast = head;
        // Find middle of list.
        while(fast->next && fast->next->next){
            slow = slow->next;
            fast =fast->next->next;
        }

        // Reverse list after middle.
        fast = head;
        while(fast->next){
            fast = fast->next;
        }
        ListNode *start, *tmp;
        start = slow->next;
        while(start != fast){
            tmp = start;
            start = start->next;
            tmp->next = fast->next;
            fast->next = tmp;
        }
        slow->next = NULL;

        //Do zigzag ordering.
        slow = head;
        fast = start;
        ListNode *dummy = new ListNode(0);
        head = dummy->next;
        while(slow || fast){
            if(slow) dummy->next = slow, slow = slow->next, dummy = dummy->next;
            if(fast) dummy->next = fast, fast = fast->next, dummy = dummy->next;
        }
    }
};