Leetcode Reorder List

Given a singly linked list L: L₀?L₁?…?L_n-1?L_n,
reorder it to: L₀?L_n?L₁?L_n-1?L₂?L_n-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}.

先通过快慢指针找到转换节点，将转换节点后半部分逆序，然后再将转换节点依次插入前半部分即可。

代码如下：

/** * 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)                return;                ListNode* p1,*p2;        p1 = p2 = head;        while(p2->next && p2->next->next)        {            p1 = p1->next;            p2 = p2->next->next;        }                ListNode* cur = p1->next;        ListNode* next;        while(cur->next)        {            next = cur->next;            cur->next=next->next;            next->next = p1->next;            p1->next = next;        }                p2 = head;        cur = p1->next;        while(p2!=p1)        {            p1->next = cur->next;            cur->next = p2->next;            p2->next = cur;            p2 = cur->next;            cur = p1->next;        }    }};