在python中以级别顺序遍历二叉树

时间:2018-02-14 15:18:53

标签: python binary-tree breadth-first-search tree-traversal

我有python script为给定的数学表达式生成二叉树。如果我在级别顺序中遍历它,我试图写一个function to print the binary tree

e.g。如果函数是((2 + 3) - 4),则输出应为

   -
  / \
 +   4
/ \
2  3

output: 
-
+ 4
2 3

将数学表达式转换为二叉树的代码

from pythonds.basic.stack import Stack
from pythonds.trees.binaryTree import BinaryTree

def buildParseTree(fpexp):
        fplist = fpexp.split()
        pStack = Stack()
        eTree = BinaryTree('')
        pStack.push(eTree)
        currentTree = eTree
        for i in fplist:
            if i == '(':
                currentTree.insertLeft('')
                pStack.push(currentTree)
                currentTree = currentTree.getLeftChild()
            elif i not in ['+', '-', '*', '/', ')']:
                currentTree.setRootVal(int(i))
                parent = pStack.pop()
                currentTree = parent
            elif i in ['+', '-', '*', '/']:
                currentTree.setRootVal(i)
                currentTree.insertRight('')
                pStack.push(currentTree)
                currentTree = currentTree.getRightChild()
            elif i == ')':
                currentTree = pStack.pop()
            else:
                raise ValueError
        return eTree

我使用标准伪代码进行广度优先搜索。

printTree(Node root)

   if(root == NULL) return

   else
      create a queue 'q'
      q.enqueue(root)

      while(q is not empty)
           root = q.dequeue
           print(root)

           if(leftChild != NULL)
              q.enqueue(leftChild)
           if(rightChild != NULL)
              q.enqueue(rightChild)

以下是我编写的用于按级别顺序打印树的python代码。

import sys

class Node:
    def __init__(self, data):
        self.data = data
        self.leftChild = None
        self.rightChid = None


class Queue:
    def __init__(self):
        self.items = []

    def isEmpty(self):
        return self.items == []

    def enqueue(self,item):
        self.items.insert(0, item)

    def dequeue(self):
        self.items.pop()

def printNodesInLevels(root):
    if root is None:
        return
    else:
        q = Queue()
        q.enqueue(root)

        while(q is not None):
            root = q.dequeue()
            print(root.getRootVal())

            if(root.leftChild is not None):
                q.enqueue(root.leftChild)

            if(root.rightChild is not None):
                q.enqueue(root.rightChild)

这是我调用函数的方式。

pt = buildParseTree("( ( 2 + 3 ) - 4 )")
printNodesInLevels(pt)

以下是我收到的错误消息。我想我没有正确地将树的根传递给打印功能。

  

     

Traceback(最近一次调用最后一次):文件   " C:/Users/YASODA/PycharmProjects/HelloWorld/BinaryTree.py" ;,第77行,   在       printNodesInLevels(pt)File" C:/Users/YASODA/PycharmProjects/HelloWorld/BinaryTree.py" ;,第67行,   在printNodesInLevels中       if(root.leftChild不是None):AttributeError:' NoneType'对象没有属性' leftChild'

1 个答案:

答案 0 :(得分:1)

您的代码中有两个错误。第一个是,你需要以队列形式返回,

class Queue:
    def __init__(self):
        self.items = []

    def isEmpty(self):
        return self.items == []

    def enqueue(self,item):
        self.items.insert(0, item)

    def dequeue(self):
        return self.items.pop()

第二个是在while循环中检查队列是否为空,

while(not q.isEmpty()):