TreeStackDFS
Print the inorder traversal of a binary tree (left subtree, node, right subtree). Do it iteratively with an explicit stack instead of recursion. Build the tree from the level-order input yourself, then solve the problem.
Input
A single line giving the tree in level order, with the word null for a missing child (for example 3 9 20 null null 15 7).
Output
Print the node values in inorder, separated by spaces.
Constraints
1 <= number of nodes <= 2000
Example 1
Input
1 null 2 3
Output
1 3 2
Inorder visits 1, then the left child of 2 (which is 3), then 2.