​LeetCode刷题实战143： 重排链表

示例 1:

给定链表 1->2->3->4, 重新排列为 1->4->2->3.

示例 2:

给定链表 1->2->3->4->5, 重新排列为 1->5->2->4->3.

/**
 * 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) return;
        ListNode *left = head, *right = head;
        while(right){
            right = right->next;
            if(right){
                right = right->next;
                left = left->next;
            }
        }
        right = left->next;
        left->next = NULL;
        left = NULL;
        ListNode *now = right;
        while(now){
            right = now->next;
            now->next = left;
            left = now;
            now = right;
        }
        now = head;
        while(left){
            right = left->next;
            left->next = now->next;
            now->next = left;
            now = left->next;
            left = right;
        }
    }
};