Program to generate tree using preorder and inorder traversal in python

Suppose we have the preorder and inorder traversal of a binary tree. We have to recover the tree from these sequences. So if the preorder and inorder sequences are [3,9,20,15,7] and [9,3,15,20,7], then the tree will be:

Let us see the steps:

  • Define a method called buildTree(), this will the preorder and inorder traversal sequences.
  • if inorder traversal list is not empty, then
    • root := create a tree node with value same first element from preorder sequence, and remove first item from preorder traversal
    • root_index := index of value of root in inorder list
    • left of root := buildTree(preorder, inorder[from index 0 to root_index])
    • right of root := buildTree(preorder, inorder[from index root_index+1 to end])
  • return root

Let us see the following implementation to get better understanding:


Live Demo

class TreeNode:
   def __init__(self, data, left = None, right = None): = data
      self.left = left
      self.right = right

def print_tree(root):
   if root is not None:
      print(, end = ',')

class Solution(object):
   def buildTree(self, preorder, inorder):
      if inorder:
         root = TreeNode(preorder.pop(0))
         root_index = inorder.index(
         root.left = self.buildTree(preorder,inorder[:root_index])
         root.right = self.buildTree(preorder,inorder[root_index+1:])
      return root

ob1 = Solution()
print_tree(ob1.buildTree([3,9,20,15,7], [9,3,15,20,7]))





Updated on: 26-Nov-2020


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