Construct Binary Tree from Inorder and Postorder Traversal in Python

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

Let us see the steps -

• Suppose the method is called buildTree with preorder and inorder lists
• root := last node from the postorder, and delete first node from postorder
• root_index := index of root.val from the inorder list
• left or root := buildTree(subset of inorder from root_index + 1 to end, postorder)
• right or root := buildTree(subset of inorder from index 0 to root_index, postorder)

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):
   #print using inorder traversal
   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([9,3,15,20,7], [9,15,7,20,3]))

Input

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

Output

9, 15, 7, 20, 3,