LeetCode143. 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 may not modify the values in the list's nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.

Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

解题思路：

1.将链表分成两部分，并且前部分与后部分相等或者大1。（快慢指针！！！）

2.将后部分反转。

3.将两部分合在一起。

/**
 * 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==NULL||head->next==NULL||head->next->next==NULL)
            return ;
        ListNode* slow=head;
        ListNode* fast=head;
        while(fast->next!=NULL) //find mid
        {
            fast=fast->next;
            if(fast->next==NULL) break;
            fast=fast->next;
            slow=slow->next;
        }
        ListNode* head2=slow->next;
        slow->next=NULL;
        ListNode* cur=head2;
        ListNode* nex=cur->next;
        cur->next=NULL;
        while(nex!=NULL)
        {
            ListNode* tmp=nex->next;
            nex->next=cur;
            cur=nex;
            nex=tmp;
        }
        head2=cur;
        ListNode* left1=head;
        ListNode* left2=head->next;
        ListNode* right1=cur;
        ListNode* right2=cur->next;
        while(right1!=NULL)
        {
            left1->next=right1;
            right1->next=left2;
            left1=left2;
            right1=right2;
            if(right2==NULL) break;
            left2=left2->next;
            right2=right2->next;
        }
    }
};