用栈来实现二叉树的非递归中序遍历
1、栈初始化：从根节点出发一路向左，将一路上的节点push到栈中
2、取next并进行栈的调整：从stack栈顶pop出来的节点即为要取的next节点。如果该节点有右孩子，则从该节点出发，往右走一步，再一路向左，将一路上的节点push进栈。
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