Python递归方法中的PreOrder遍历

Question

Why is this struck in loop,Can you please help me

class createTreeNode():

    def __init__(self,value):
        self.value=value
        self.left=None
        self.right=None


def preOrderTraversal(root):

    while(root):
        print root.value
        preOrderTraversal(root.left)
        preOrderTraversal(root.right)

    return
def insertNodetoBST(root,i):

    if(root==None):
        return createTreeNode(i)
    temproot=root;
    while(temproot):
        if(temproot.value>i):
            if(temproot.left==None):
                temproot.left=createTreeNode(i)
                return root
            temproot = temproot.left
        else:
            if (temproot.right == None):
                temproot.right = createTreeNode(i)
                return root
            temproot=temproot.right
    return root



def createBSTfromList():

    root=None
    l = [1,2]
    for i in l:
        root=insertNodetoBST(root,i);
    preOrderTraversal(root);

createBSTfromList();

Answer 1

Your code loops forever because you have while(root) in your preOrderTraversal function. Since you're not modifying root to anything falsey during the loop body, this will always run forever if root was not None to start with.

I think you want that line to be if root instead. This way you'll run the block either zero (base case) or one times (recursive case):

def preOrderTraversal(root):
    if root:
        print root.value
        preOrderTraversal(root.left)
        preOrderTraversal(root.right)

You don't need parentheses around the condition of an if or while statement. You also don't need any of the semicolons you're putting at the end of your lines. You may want to read PEP 8 which is a style-guide for Python code.