Reorder List

<-M 143> Reorder List

/**
 * 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 == nullptr || head->next == nullptr)
            return;

        ListNode * slow = head, *fast = head, *prev = nullptr;
        while(fast && fast->next) {
            prev = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        prev->next = nullptr;
        slow = reverse(slow);

        ListNode *cur = head;
        while(cur->next) {
            ListNode *temp = cur->next;
            cur->next = slow;
            slow = slow->next;
            cur->next->next = temp;
            cur = temp;
        }
        cur->next = slow;
    }
private:
    ListNode *reverse(ListNode *head) {
        if(head == nullptr || head->next == nullptr)
            return head;

        ListNode *prev = head;
        for(ListNode *cur = head->next, *next = cur->next;
           cur;
           prev = cur, cur = next, next = next ? next->next : nullptr) {
            cur->next = prev;
        }

        head->next = nullptr;
        return prev;
    }
};