leetcode-143. Reorder List

/**
 * 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;
        //用快慢指针将链表分为两部分，若为奇数则第一部分大1
        ListNode *fast,*slow,*head1,*head2,*post,*cur,*tmp = NULL;
        fast = slow = head;
        while(fast->next != NULL)
        {
            fast = fast->next;
            if(fast->next == NULL) break;
            fast = fast->next;
            slow = slow->next;
        }
        head1 = head;
        cur = head2 = slow->next;
        slow->next = NULL;  //这里要注意，断开的部分要设NULL
        //将第二部分链表反转
        post = head2->next;
        cur->next = NULL;
        while(post!=NULL)
        {
            tmp = post->next;
            post->next = cur;
            cur = post;
            post = tmp;
        }
        head2 = cur;
        //将第二部分插入到第一部分
        ListNode *cur1 = head1;
        ListNode *cur2 = head2;
        while(cur2!=NULL)
        {
            tmp = cur1->next;
            cur1->next = cur2;
            post = cur2->next;
            cur2->next = tmp;
            cur1 = tmp;
            cur2 = post;
        }
    }
};