Given the head of a singly linked list and an integer k, return the value of the k-th node counting from 1 (so k = 1 is the head). If there is no such node (k is less than 1 or larger than the length), return None.
Examples
Input: head = 10 -> 20 -> 30, k = 2
Output: 20
Input: head = 10 -> 20 -> 30, k = 4
Output: None
Constraints
0 <= number of nodes <= 10**4-10**4 <= k <= 10**4- Target: O(k) time, O(1) extra space; do not build a Python list of the values
Goals
- Advance a pointer a fixed number of steps
- Return None when the position does not exist