Syntax error Program to generate tree using preorder and inorder traversal in python

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:

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,
Updated on: 2020-11-26T07:51:32+05:30

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