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Reorder List

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Description

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

Solution

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* ret = reverse(head->next);
        head->next->next = head;
        head->next = NULL;
        return ret;
    }
    
    void reorderList(ListNode* head) {
        if (!head || !head->next) return;
        
        ListNode* slow = head;
        ListNode* quick = head;
        while(quick && quick->next && quick->next->next) {
            slow = slow->next;
            quick = quick->next->next;
        }
        ListNode* newHead = reverse(slow->next);
        slow->next = NULL;
        
        //Merge crossly from head and newHead;
        ListNode* cur = head;
        ListNode* another = newHead;
        while (cur && another) {
            ListNode* temp = cur;
            cur = cur->next;
            temp->next = another;
            temp = another;
            another = another->next;
            temp->next = cur;
        }
    }
};