Construct Binary Tree from Preorder and Inorder Traversal in Python


Suppose we have the inorder and preorder traversal sequence of a binary tree. We have to generate 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 −

  • Suppose the method is called buildTree with preorder and inorder lists

  • root := first node from the preorder, and delete first node from preorder

  • root_index := index of root.val from the inorder list

  • left or root := buildTree(preorder, subset of inorder from 0 to root_index)

  • right or root := buildTree(preorder, subset of inorder from root_index + 1 to end)

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

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

Output

[3,9,20,null,null,15,7]
raja
Published on 03-Feb-2020 10:33:03
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