TreeBSTDFS
Given a valid binary search tree and an integer k, find the k-th smallest value (counting from 1) stored in it. Build the tree from the level-order input yourself, then solve the problem.
Input
Line 1: the tree in level order with null for missing children. Line 2: k.
Output
Print the k-th smallest value.
Constraints
1 <= k <= number of nodes <= 2000 The tree is a valid BST with distinct values.
Example 1
Input
3 1 4 null 2 1
Output
1
The smallest value in the tree is 1.
Example 2
Input
5 3 6 2 4 null null 1 3
Output
3
Inorder is 1 2 3 4 5 6, so the 3rd is 3.