708. Insert into a Cyclic Sorted List

https://www.lintcode.com/problem/insert-into-a-cyclic-sorted-list/description

有序的环形单指针列表在保持有序的前提下，插入给定新结点x。把列表看成从小到大有序排列，一共有以下这些情况：1. 插入的点在中间，即cur->val <= x && x <= next->val 2. 在末尾next->val < cur->val && cur->val <= x 3. 在开头，next->val < cur->val && x <= next->val。遍历时遇到这三种情况后break并插入。为了防止遍历陷入死循环（如2->2->2），检查cur!=head。

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */


class Solution {
public:
    /*
     * @param node: a list node in the list
     * @param x: An integer
     * @return: the inserted new list node
     */
    ListNode * insert(ListNode * head, int x) {
        ListNode *cur = head, *res = new ListNode(x), *next = NULL;
        if (head == NULL) {
            res->next = res;
            return res;
        }
        next = cur->next;
        while (cur->next != head) {
            if (next->val < cur->val && (cur->val <= x || x <= next->val)) {
                    break;
            }
            if (cur->val <= x && x <= next->val) {
                break;
            }
            cur = cur->next;
            next = cur->next;
        }
        cur->next = res;
        res->next = next;
        return res;
    }
};