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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)
Let us see the following implementation to get better understanding −
Example
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([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,
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