小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
迭代法来遍历二叉搜索树，用递归大家都可以实现对二叉搜索树的遍历，其实递归等同于栈，接下来介绍如何用迭代方式来实现对二叉搜索树的遍历。
第一种迭代方式遍历
将根节点压入栈后出栈，将根节点值添加到数组，因为栈是先进后出，搜索在前序(中左右）是先将右侧子节点压入栈再将左侧子结点压入栈，这样出栈顺序就是左结点和右结点。
def _preorder_iterator(self,cur_node,res):
stack = []
stack.append(cur_node)
while len(stack) >0:
cur_node = stack.pop()
res.append(cur_node.val)
if cur_node.right != None:
stack.append(cur_node.right)
if cur_node.left != None:
stack.append(cur_node.left)
第二种迭代方式遍历
这种方式先是从根节点一路将其左结点压入栈，并且将这些值添加到返回数组，随后出栈时并将其右子节点
def _preorder_iterator(self,cur_node,res):
stack = []
stack.append(cur_node)
while len(stack) >0:
cur_node = stack.pop()
res.append(cur_node.val)
if cur_node.right != None:
stack.append(cur_node.right)
if cur_node.left != None:
stack.append(cur_node.left)
class Node:
def __init__(self,val):
self.val = val
self.left = None
self.right = None
class BinarySearchTree:
def __init__(self):
self.root = None
def insert(self,val):
if self.root == None:
self.root = Node(val)
else:
self._insert(val,self.root)
def _insert(self,val,cur_node):
if val > cur_node.val:
if cur_node.right == None:
cur_node.right = Node(val)
else:
self._insert(val,cur_node.right)
elif val < cur_node.val:
if cur_node.left == None:
cur_node.left = Node(val)
else:
self._insert(val,cur_node.left)
def traversal(self):
res = []
self._preorder_iterator(self.root,res)
return res
def _preorder_iterator(self,cur_node,res):
stack = []
stack.append(cur_node)
while len(stack) >0:
cur_node = stack.pop()
res.append(cur_node.val)
if cur_node.right != None:
stack.append(cur_node.right)
if cur_node.left != None:
stack.append(cur_node.left)
def _preorder(self,cur_node,res):
if cur_node == None: return
res.append(cur_node.val)
self._preorder(cur_node.left,res)
self._preorder(cur_node.right,res)
if __name__ == "__main__":
bst = BinarySearchTree()
bst.insert(5)
bst.insert(3)
bst.insert(4)
bst.insert(1)
bst.insert(2)
print(bst.traversal())
