Reorder_List.cpp

/**
 * 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:
ListNode *reverse(ListNode* head){
    if(!head)
        return NULL;
    ListNode *curr = head, *prev = NULL;
    while(curr){
        ListNode *tmp = curr->next;
        curr->next = prev;
        prev = curr;
        curr = tmp;
    }
    return prev;
}

void reorderList(ListNode* head) {
    if(!head || !head->next)
        return;
    ListNode *fast = head, *slow = head, *tmp;
    while(fast->next && fast->next->next){
        fast = fast->next->next;
        slow = slow->next;
    }
    tmp = slow->next;
    slow->next = NULL;
    fast = reverse(tmp);
    slow = head;
    while(slow && fast){
        tmp = slow->next;
        slow->next = fast;
        fast = fast->next;
        slow = slow->next;
        slow->next = tmp;
        slow = slow->next;
    }
}
};