博主曾在之前的博客中用 python實現樹結構,此篇博客將專門用 Python 實現樹的各種遞歸和非遞歸的遍歷。
1. 構建樹
我們先構建一棵簡單的樹:
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
a = TreeNode(1)
b = TreeNode(2)
c = TreeNode(3)
d = TreeNode(4)
e = TreeNode(5)
f = TreeNode(6)
g = TreeNode(7)
a.left = b
a.right = c
b.left = d
b.right = e
c.left = f
c.right = g
這是一棵這樣的樹:
2. 前序遍歷
根節點->左子樹->右子樹
# 先序打印二叉樹(遞歸)
def preOrderTraverse(node):
if not node:
return None
print(node.val)
preOrderTraverse(node.left)
preOrderTraverse(node.right)
# 先序打印二叉樹(非遞歸)
def preOrderTravese(node):
stack = [node]
while len(stack) > 0:
print(node.val)
if node.right:
stack.append(node.right)
if node.left:
stack.append(node.left)
node = stack.pop()
輸出:1245367
2. 中序遍歷
左子樹->根節點->右子樹
# 中序打印二叉樹(遞歸)
def inOrderTraverse(node):
if node is None:
return None
inOrderTraverse(node.left)
print(node.val)
inOrderTraverse(node.right)
# 中序打印二叉樹(非遞歸)
def inOrderTraverse(node):
stack = []
pos = node
while pos or len(stack) > 0:
if pos:
stack.append(pos)
pos = pos.left
else:
pos = stack.pop()
print(pos.val)
pos = pos.right
輸出:4251637
4. 後序遍歷
左子樹->右子樹->根節點
# 後序打印二叉樹(遞歸)
def postOrderTraverse(node):
if node is None:
return None
postOrderTraverse(node.left)
postOrderTraverse(node.right)
print(node.val)
# 後序打印二叉樹(非遞歸)
# 使用兩個棧結構
# 第一個棧進棧順序:左節點->右節點->跟節點
# 第一個棧彈出順序: 跟節點->右節點->左節點(先序遍歷棧彈出順序:跟->左->右)
# 第二個棧存儲爲第一個棧的每個彈出依次進棧
# 最後第二個棧依次出棧
def postOrderTraverse(node):
stack = [node]
stack2 = []
while len(stack) > 0:
node = stack.pop()
stack2.append(node)
if node.left is not None:
stack.append(node.left)
if node.right is not None:
stack.append(node.right)
while len(stack2) > 0:
print(stack2.pop().val)
輸出:4526731
5. 層次遍歷
逐層遍歷
def layerTraverse(node):
if not node:
return None
queue = []
queue.append(node)
while len(queue) > 0:
tmp = queue.pop(0)
print(tmp.val)
if node.left:
queue.append(tmp.left)
if node.right:
queue.append(tmp.right)
輸出:1234567