如何在Python中建立二叉树

Question

I am trying to build a binary tree, but stuck at adding two nodes to root.

What function should I write to enable adding node to these two nodes?

I have the following code:

class Btree:
        def __init__(self, root):
                self.key = root
                self.lc = None
                self.rc = None

        def insert_lc(self, newNode):
                if self.lc == None:
                        self.lc = Btree(newNode)
                else:
                        t = Btree(newNode)
                        t.lc = self.lc
                        self.lc = t
        def insert_rc(self, newNode):
                if self.rc == None:
                        self.rc = Btree(newNode)
                else:
                        t = Btree(newNode)
                        t.rc = self.rc
                        self.rc = t

        def get_rc(self):
                return self.rc
        def get_lc(self):
                return self.lc

        def set_Root(self, val):
                self.key = val
        def get_Root(self):
                return self.key

r = Btree(1)
r.insert_lc(2)
r.insert_rc(4)

I think I need a function so I can add left child and right child to nodes which have value 2 and 4

Answer 1

First of all, I do not quite understand why do you assign your new node's left (right) child to that new node itself. This will get you an infinite loop if you try to search the tree later.

t = Btree(newNode)
#The line below seems unnecessary
t.lc = self.lc
self.lc = t

Second of all, in the Btree constructor you have a parameter called root which seems like it's intended to be a reference to the root of the current node. However, you pass the node's key as that parameter, so there is no way to access node's root. I understand that is might be not even necessary in your case, still that parameter name is quite confusing. If you want to have a reference to the parent node you should add another parameter to your constructor:

def __init__(self, root, key):
    self.root = root
    self.key = key
    self.lc = None
    self.rc = None

This way you have both key and a parent node reference. You also will need to modify insert_rc and insert_lc methods:

def insert_rc(self, newNode):
    if self.rc == None:
        #Parent node is the one the insert_rc method is called on,
        #so we pass self as the root parameter
        self.rc = Btree(self, newNode)
    else:
        t = Btree(self, newNode)
        self.rc = t
#Same with insert_lc

Finally,to answer your initial question: to add children to the added nodes you simply call get_rc or get_lc both of which will return you instances of Btree so you can add child nodes to them:

r = Btree(1)
r.insert_lc(2)
r.insert_rc(4)
#Insert left child to the node with key=2
r.get_lc().insert_lc(3)
#Insert right child to the node with key=4
r.get_rc().insert_rc(5)