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:

Example

Live Demo

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

def print_tree(root):
   if root is not None:
      print_tree(root.left)
      print(root.data, end = ',')
      print_tree(root.right)

class Solution(object):
   def buildTree(self, preorder, inorder):
      if inorder:
         root = TreeNode(preorder.pop(0))
         root_index = inorder.index(root.data)
         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]))

Input

[3,9,20,15,7],[9,3,15,20,7]

Output

9,3,15,20,7,