[LeetCode] 1586. Binary Search Tree Iterator II

 Problem : https://leetcode.com/problems/binary-search-tree-iterator-ii/

Use another stack to save the previous nodes while iterating through in-order traversal.

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class BSTIterator:

    def __init__(self, root: Optional[TreeNode]):
        self.stack = []
        self.pre = []
        
        while root:
            tmp = root.left
            root.left = None
            self.stack.append(root)
            root = tmp
            

    def hasNext(self) -> bool:
        return len(self.stack) > 0
        
    def next(self) -> int:
        node = self.stack.pop()
        self.pre.append(node)
        
        if node.right:
            tmp = node.right
            node.right = None
            
            node = tmp
            
            while node:
                tmp = node.left
                node.left = None
                self.stack.append(node)
                node = tmp
        
        return self.pre[-1].val
    
    def hasPrev(self) -> bool:
        return len(self.pre) >= 2
        

    def prev(self) -> int:
        node = self.pre.pop()
        self.stack.append(node)
        return self.pre[-1].val


# Your BSTIterator object will be instantiated and called as such:
# obj = BSTIterator(root)
# param_1 = obj.hasNext()
# param_2 = obj.next()
# param_3 = obj.hasPrev()
# param_4 = obj.prev()