题目地址：

https://www.lintcode.com/problem/insert-node-in-sorted-linked-list/description

给定一个有序链表，向其插入一个数使其仍然保持有序。思路很简单，只需要先寻找要插入的位置即可。代码如下：

public class Solution {
    /**
     * @param head: The head of linked list.
     * @param val: An integer.
     * @return: The head of new linked list.
     */
    public ListNode insertNode(ListNode head, int val) {
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode cur = dummy;
        
        while (cur.next != null && cur.next.val < val) {
            cur = cur.next;
        }
        
        ListNode insertion = new ListNode(val);
        insertion.next = cur.next;
        cur.next = insertion;
        
        return dummy.next;
    }
}

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) {
        val = x;
        next = null;
    }
}

时间复杂度 $O(n)$，空间 $O(1)$。