Note: 存在两种情况，插入的新node插在头，或插在中间

public class Solution {
  public ListNode insert(ListNode head, int value) {
    ListNode newNode = new ListNode(value);
    if (head == null || head.value > value) {
      newNode.next = head;
      return newNode;
    }
    ListNode cur = head;
    while (cur.next != null && cur.next.value < value) {
      cur = cur.next;
    }
    ListNode next = cur.next;
    cur.next = newNode;
    newNode.next = next;
    return head;
  }
}

Complexity Analysis:

TC: O(n)

SC: O(1)