Python中的树
请帮助我理解Python中的树。这是我在互联网上找到的树实现的一个例子Python中的树,python,tree,Python,Tree,请帮助我理解Python中的树。这是我在互联网上找到的树实现的一个例子 from collections import deque class EmptyTree(object): """Represents an empty tree.""" # Supported methods def isEmpty(self): return True def __str__(self): return "" def __iter_
from collections import deque
class EmptyTree(object):
"""Represents an empty tree."""
# Supported methods
def isEmpty(self):
return True
def __str__(self):
return ""
def __iter__(self):
"""Iterator for the tree."""
return iter([])
def preorder(self, lyst):
return
def inorder(self, lyst):
return
def postorder(self, lyst):
return
class BinaryTree(object):
"""Represents a nonempty binary tree."""
# Singleton for all empty tree objects
THE_EMPTY_TREE = EmptyTree()
def __init__(self, item):
"""Creates a tree with
the given item at the root."""
self._root = item
self._left = BinaryTree.THE_EMPTY_TREE
self._right = BinaryTree.THE_EMPTY_TREE
def isEmpty(self):
return False
def getRoot(self):
return self._root
def getLeft(self):
return self._left
def getRight(self):
return self._right
def setRoot(self, item):
self._root = item
def setLeft(self, tree):
self._left = tree
def setRight(self, tree):
self._right = tree
def removeLeft(self):
left = self._left
self._left = BinaryTree.THE_EMPTY_TREE
return left
def removeRight(self):
right = self._right
self._right = BinaryTree.THE_EMPTY_TREE
return right
def __str__(self):
"""Returns a string representation of the tree
rotated 90 degrees to the left."""
def strHelper(tree, level):
result = ""
if not tree.isEmpty():
result += strHelper(tree.getRight(), level + 1)
result += " " * level
result += str(tree.getRoot()) + "\n"
result += strHelper(tree.getLeft(), level + 1)
return result
return strHelper(self, 0)
def __iter__(self):
"""Iterator for the tree."""
lyst = []
self.inorder(lyst)
return iter(lyst)
def preorder(self, lyst):
"""Adds items to lyst during
a preorder traversal."""
lyst.append(self.getRoot())
self.getLeft().preorder(lyst)
self.getRight().preorder(lyst)
def inorder(self, lyst):
"""Adds items to lyst during
an inorder traversal."""
self.getLeft().inorder(lyst)
lyst.append(self.getRoot())
self.getRight().inorder(lyst)
def postorder(self, lyst):
"""Adds items to lystduring
a postorder traversal."""
self.getLeft().postorder(lyst)
self.getRight().postorder(lyst)
lyst.append(self.getRoot())
def levelorder(self, lyst):
"""Adds items to lyst during
a levelorder traversal."""
# levelsQueue = LinkedQueue()
levelsQueue = deque ([])
levelsQueue.append(self)
while levelsQueue != deque():
node = levelsQueue.popleft()
lyst.append(node.getRoot())
left = node.getLeft()
right = node.getRight()
if not left.isEmpty():
levelsQueue.append(left)
if not right.isEmpty():
levelsQueue.append(right)
这是制作小树的程序
"""
File: testbinarytree.py
Builds a full binary tree with 7 nodes.
"""
from binarytree import BinaryTree
lst = ["5", "+", "2"]
for i in range(len(lst)):
b = BinaryTree(lst[0])
d = BinaryTree(lst[1])
f = BinaryTree(lst[2])
# Build the tree from the bottom up, where
# d is the root node of the entire tree
d.setLeft(b)
d.setRight(f)
def size(tree):
if tree.isEmpty():
return 0
else:
return 1 + size(tree.getLeft()) + size(tree.getRight())
def frontier(tree):
"""Returns a list containing the leaf nodes
of tree."""
if tree.isEmpty():
return []
elif tree.getLeft().isEmpty() and tree.getRight().isEmpty():
return [tree.getRoot()]
else:
return frontier(tree.getLeft()) + frontier(tree.getRight())
print ("Size:", size(d))
print ("String:")
print (d)
如何创建一个类来计算表达式的值,从而使答案=7(5+2)。我真的想用一个小例子来理解这个概念。你应该做一个函数,按深度顺序遍历一棵树,计算每个节点的值,或者只取它的值(例如,如果它是“5”),或者进行计算(例如,如果它是“+”)-通过按深度优先顺序遍历树,您可以确保在计算给定节点时将计算该节点的所有子节点(例如,在计算“+”时将计算“5”和“2”)
然后,在树的根上,你会得到整棵树的结果。首先,如果这是家庭作业,我不会给出太多细节,听起来有点像 在树类上需要一个计算树的方法。我假设每个树节点的“根”值是一个数字(当节点是叶子时,即没有子节点时)或操作符的名称(当节点有子节点时) 您的方法将是递归的:具有子节点的树节点的值由(1)其左子树的值、(2)其右子树的值和(3)其“根”中的运算符确定
您可能需要一个表——可能存储在一个
dict
——将“+”
之类的运算符名称映射到运算符之类的实际函数。添加(或者,如果您愿意,lambda x,y:x+y
)。听起来您的问题不是树,树是一个更一般(更简单)的概念,但是如何正确地填充和/或计算表达式树
如果在修复后的顺序中指定了运算符,那么就容易多了
看。它被称为调车场算法。“计算表达式的值”。什么意思?是否要创建表达式计算器(可能是计算器),将其用作语法树?哪些部分你懂,哪些部分你不懂?是的,我希望它能像计算器一样工作。我理解上面写的一切。我所有试图编写用于计算表达式的类的尝试都以失败告终。它不是真正的中缀!给定的代码手动“解析”表达式,并建立一个树,树的叶子上有5和2,根上有“+”
。“我不知道提问者在处理更复杂的表达方面有什么野心……”加雷斯:我把这个问题转移到了一个答案上,因为我意识到这可能会给他所需要的一切。