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Python Construct BST from given preorder traversal

DarkDevilor answered on November 10, 2022 Popularity 9/10 Helpfulness 1/10

answer Python Construct BST from given preorder traversal

Python Construct BST from given preorder traversal

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# A O(n^2) Python3 program for
# construction of BST from preorder traversal
 
# A binary tree node
 
 
class Node():
 
    # A constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
 
 
# constructTreeUtil.preIndex is a static variable of
# function constructTreeUtil
 
# Function to get the value of static variable
# constructTreeUtil.preIndex
def getPreIndex():
    return constructTreeUtil.preIndex
 
# Function to increment the value of static variable
# constructTreeUtil.preIndex
 
 
def incrementPreIndex():
    constructTreeUtil.preIndex += 1
 
# A recurseive function to construct Full from pre[].
# preIndex is used to keep track of index in pre[[].
 
 
def constructTreeUtil(pre, low, high):
 
        # Base Case
    if(low > high):
        return None
 
    # The first node in preorder traversal is root. So take
    # the node at preIndex from pre[] and make it root,
    # and increment preIndex
    root = Node(pre[getPreIndex()])
    incrementPreIndex()
 
    # If the current subarray has onlye one element,
    # no need to recur
    if low == high:
        return root
 
    r_root = -1
 
    # Search for the first element greater than root
    for i in range(low, high+1):
        if (pre[i] > root.data):
            r_root = i
            break
 
    # If no elements are greater than the current root,
    # all elements are left children
    # so assign root appropriately
    if r_root == -1:
        r_root = getPreIndex() + (high - low)
 
    # Use the index of element found in preorder to divide
    # preorder array in two parts. Left subtree and right
    # subtree
    root.left = constructTreeUtil(pre, getPreIndex(), r_root-1)
 
    root.right = constructTreeUtil(pre, r_root, high)
 
    return root
 
# The main function to construct BST from given preorder
# traversal. This function mailny uses constructTreeUtil()
 
 
def constructTree(pre):
    size = len(pre)
    constructTreeUtil.preIndex = 0
    return constructTreeUtil(pre, 0, size-1)
 
 
def printInorder(root):
    if root is None:
        return
    printInorder(root.left)
    print (root.data,end=' ')
    printInorder(root.right)
 
 
# Driver code
pre = [10, 5, 1, 7, 40, 50]
 
root = constructTree(pre)
print ("Inorder traversal of the constructed tree:")
printInorder(root)

Popularity 9/10 Helpfulness 1/10 Language python

Source: www.geeksforgeeks.org

Tags: construct preorder python traversal

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Contributed on Nov 10 2022

DarkDevilor

0 Answers Avg Quality 2/10

Python Construct BST from given preorder traversal

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