Binary Search Tree Iterator 2

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1. Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST):

2. BSTIterator(TreeNode root) Initializes an object of the BSTIterator class. The root of the BST is given as part of the constructor. The pointer should be initialized to a non-existent number smaller than any element in the BST.
boolean hasNext() Returns true if there exists a number in the traversal to the right of the pointer, otherwise returns false.
int next() Moves the pointer to the right, then returns the number at the pointer.

3. You may assume that next() calls will always be valid. That is, there will be at least a next number in the in-order traversal when next() is called.
Input Format
Input is managed for you.
Output Format
Output is managed for you. 
Question Video
1. The number of nodes in the tree is in the range [1, 105].
2. 0 <= Node.val <= 106
3. At most 105 calls will be made to hasNext, and next.
Sample Input
1 2 3 4 5 6 7
Sample Output

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