708. Insert into a Sorted Circular Linked List

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LeetCode

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Problem

Given a node from a Circular Linked List which is sorted in ascending order, write a function to insert a value insertVal into the list such that it remains a sorted circular list. The given node can be a reference to any single node in the list, and may not be necessarily the smallest value in the circular list.

If there are multiple suitable places for insertion, you may choose any place to insert the new value. After the insertion, the circular list should remain sorted.

If the list is empty (i.e., given node is null), you should create a new single circular list and return the reference to that single node. Otherwise, you should return the original given node.

Example 1:

Input: head = [3,4,1], insertVal = 2
Output: [3,4,1,2]
Explanation: In the figure above, there is a sorted circular list of three elements. You are given a reference to the node with value 3, and we need to insert 2 into the list. The new node should be inserted between node 1 and node 3. After the insertion, the list should look like this, and we should still return node 3.

Example 2:

Input: head = [], insertVal = 1
Output: [1]
Explanation: The list is empty (given head is null). We create a new single circular list and return the reference to that single node.

Example 3:

Input: head = [1], insertVal = 0
Output: [1,0]

Constraints:

• The number of nodes in the list is in the range $[0, 5 * 10^4]$.
• $-10^6 <= Node.val, insertVal <= 10^6$

Code

/*
// Definition for a Node.
class Node {
    public int val;
    public Node next;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, Node _next) {
        val = _val;
        next = _next;
    }
};
*/

class Solution {
    public Node insert(Node head, int insertVal) {
        if (head == null) {
            head = new Node(insertVal);
            head.next = head;
            return head;
        }

        Node curr = head;

        // [1] 0
        while (curr.next != head) {
            // 1. insertVal在升序数组中间
            // [3,5] 4 或者 [3,3,3] 3 找到中间位置
            if (curr.val <= insertVal && curr.next.val >= insertVal) break;

            // 2. insertVal是新的最小值或最大值
            // [3,4,5] 10 或者 [3,4,5] 1
            // 此时需要找到升序数组的断点
            // prev.val > curr.val 表示当前位置是最大值和最小值之间, 重要节点!!
            if (curr.val > curr.next.val && (insertVal >= curr.val || insertVal <= curr.next.val)) break;

            curr = curr.next;
        }

        curr.next = new Node(insertVal, curr.next);
        return head;
    }
}