# 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)

## Example

Let us see the following implementation to get better understanding −

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

## Input

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

## Output

[3,9,20,null,null,15,7]
Published on 03-Feb-2020 10:35:43