python實現 二叉樹的前序中序後序遍歷層次遍歷——遞迴與非遞迴
阿新 • • 發佈:2019-01-07
前序遍歷
# ------ coding:utf-8 -------
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def pre_order_recursive(root_node):
if not root_node:
return
print root_node.val
pre_order_recursive(root_node.left)
pre_order_recursive(root_node.right)
def pre_order_non_recursive(root_node):
if not root_node:
return
node_stack = []
node = root_node
while node_stack or node:
# 從跟節點開始,把它的左子樹找出來
while node:
print node.val
node_stack.append(node)
node = node.left
# 上面 while 的結束就是 node 為空的時候,也就是前一個節點沒有左子樹的了
node = node_stack.pop()
# 這個時候就開始檢視右子樹了
node = node.right
中序遍歷
# ------ coding:utf-8 -------
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def mid_order_recursive(root_node):
if not root_node:
return
mid_order_recursive(root_node.left)
print root_node.val
mid_order_recursive(root_node.right)
def mid_order_non_recursive(root_node):
if not root_node:
return
node_stack = []
node = root_node
while node_stack or node:
# 從跟節點開始,把它的左子樹找出來
while node:
node_stack.append(node)
node = node.left
# 上面 while 的結束就是 node 為空的時候,也就是前一個節點沒有左子樹的了
node = node_stack.pop()
print node.val
# 這個時候就開始檢視右子樹了
node = node.right
後序遍歷
# ------ coding:utf-8 -------
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def post_order_recursive(root_node):
if not root_node:
return
post_order_recursive(root_node.left)
post_order_recursive(root_node.right)
print root_node.val
def post_order_non_recursive(root_node):
if not root_node:
return
stack1 = [root_node]
stack2 = []
while stack1:
# 在這個 while 迴圈裡面找到後序遍歷的逆序,存在在 stack2 中
node = stack1.pop()
if node.left:
stack1.append(node.left)
if node.right:
stack1.append(node.right)
stack2.append(node)
while stack2:
print stack2.pop().val
層次遍歷
# ------ coding:utf-8 -------
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def level_order(root_node):
if not root_node:
return
node_queue = [root_node]
while node_stack:
# python的list中的pop函式,可以傳參,指定pop哪個位置的元素,預設最後一個
node = node_queue.pop(0)
print node.val
if node.left:
node_queue.append(node.left)
if node.right:
node_queue.append(node.right)