用棧來實現二叉樹的非遞歸中序遍歷

1、棧初始化：從根節點出發一路向左，將一路上的節點push到棧中

2、取next並進行棧的調整：從stack棧頂pop出來的節點即為要取的next節點。如果該節點有右孩子，則從該節點出發，往右走一步，再一路向左，將一路上的節點push進棧。

class BSTIterator:
    """
    @param: root: The root of binary tree.
    """
    def __init__(self, root):
        self.stack = []
        while root != None:
            self.stack.append(root)
            root 
 
= root.left

    """
    @return: True if there has next node, or false
    """
    def hasNext(self):
        return len(self.stack) > 0

    """
    @return: return next node
    """
    def next(self):
        node = self.stack.pop()
        cur = node.right
        while cur is not
 
 None:
            self.stack.append(cur)
            cur = cur.left
        return node

二叉樹BST叠代器（非遞歸中序遍歷）——Binary Search Tree Iterator