LeetCode | Reorder List

https://leetcode.com/problems/reorder-list/

先找链表中点，然后把右边反转，再按要求重构链表。

class Solution {
public:
    void reorderList(ListNode* head) {
        if (!head || !head->next) return;
        
        ListNode *mid = find_mid(head);
        ListNode *right = mid->next; mid->next = NULL;
        right = reverse_list(right);
        
        ListNode dummy(0), *cur = &dummy;
        bool is_odd = true;
        while (head && right) {
            if (is_odd) { cur->next = head; head = head->next; }
            else        { cur->next = right; right = right->next; }
            cur = cur->next;
            is_odd = !is_odd;
        }
        cur->next = head ? head : right;
        return;
    }
    
    ListNode* find_mid(ListNode* head) {
        ListNode *slow = head, *fast = head->next;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
    
    ListNode* reverse_list(ListNode* head) {
        ListNode *prev = NULL;
        while (head) {
            ListNode *next = head->next;
            head->next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
};