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How to construct the python binary tree from the preorder and the preorder traversal sequence, many novices are not very clear about this. In order to help you solve this problem, the following editor will explain it in detail. People with this need can come and learn. I hope you can get something.

[title]

A binary tree is constructed according to the preorder traversal and intermediate order traversal of a tree.

Note: you can assume that there are no duplicate elements in the tree.

For example, give the

Preorder traversal preorder = [3pm 9pm 20je 15rem 7]

Traversing in the middle order inorder = [9 ~ 1 ~ 3 ~ 15 ~ 20 ~ 7]

Return the following binary tree:

three

/\

9 20

/\

15 7

[ideas]

First of all, review the order of traversal: the preorder traversal is the root node-left subtree-right subtree, and the middle order traversal is the left subtree-root node-right subtree.

Then the first element of the preorder traversal array must be the root node. If you find this element in the preorder traversal array, the first part is the element of the left subtree and the second part is the element of the right subtree. It can be solved by recursion.

Note: pre-order traversal + post-order traversal, can not determine the only binary tree!

[code]

Python version

# Definition for a binary tree node.

# class TreeNode (object):

# def _ _ init__ (self, x):

# self.val = x

# self.left = None

# self.right = None

Class Solution (object):

Def buildTree (self, preorder, inorder):

"

: type preorder: List [int]

: type inorder: List [int]

: rtype: TreeNode

"

# preorder traversal, the first one is head

# traversal in the middle order, the first part is the left subtree and the second part is the right subtree

If len (preorder) = = 0:

Return None

Node = TreeNode (preorder [0])

Index = inorder.index (preorder [0])

Node.left = self.buildTree (preorder [1: index + 1], inorder [: index])

Node.right = self.buildTree (preindex [index + 1:], inorder [index + 1:])

Return node

[similar topic]

Construction of binary Tree from Middle order and Post order ergodic sequences

The idea of solving the problem: the last element of the post-order traversal array is the element of the root node, which is also found in the order traversal array and recursively generates a binary tree.

Construction of binary tree based on preorder and postorder traversal

The idea of solving the problem: directly generate a binary tree with only the right child to meet the conditions.

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