【Leetcode】【Medium】Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

解题思路：

设置一个指针mid指向链表的中间结点；

将mid之后的半个链表进行反转，反转后的链表insert;

将insert中的结点挨个插入前半段链表中；

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode* head) {
        if (!head || !head->next || !head->next->next)
            return;
        ListNode* ctrl = head;
        ListNode* mid = head;
        ListNode* insert = NULL;
        
        while (ctrl->next && ctrl->next->next) {
            ctrl = ctrl->next->next;
            mid = mid->next;
        }
        
        insert = reverseList(mid->next);
        mid->next = NULL;
        ctrl = head;
        
        while (insert) {
            ListNode* t = ctrl->next;
            ctrl->next = insert;
            insert = insert->next;
            ctrl->next->next = t;
            ctrl = t;
        }
        
        return;
    }
    
    ListNode* reverseList(ListNode* head) {
        if (!head->next)
            return head;
        ListNode* cur = NULL;
        ListNode* next = head;
        ListNode* left = NULL;
        while (next) {
            left = next->next;
            next->next = cur;
            cur = next;
            next = left;
        }
        return cur;
    }
};