Brush reconstruction offer to prove safety record of the binary tree problem

1. Title Description

And enter the result in a preorder traversal of a binary tree in preorder traversal of the binary tree a rebuild. Suppose Results preorder traversal order and input of duplicate numbers are free. Before entering e.g. preorder traversal sequence {1,2,4,7,3,5,6,8} and {4,7,2,1,5,3,8,6} order traversal sequence, and the reconstructed binary tree return.

2. Recursive problem-solving

2.1 Analysis

Preorder traversal: root → → left and right
preorder: root → → left and right

• Sequence preorder traversal by the pre = {1,2,4,7,3,5,6,8} is a known root node;
• Then in order traversal sequence in = {4,7,2,1,5,3,8,6} 1 found, they will know the location of a left subtree is the left, the right is the right location 1 subtree can be obtained and the length of the left subtree right subtree, respectively 3 and 4;
• Recursive call: The preorder the root node in the left subtree and right subtree, respectively, as a tree, left subtree and right subtree respective nodes and then to find a preorder traversal, the left subtree of the root node found.
• In the left subtree example, {4,7,2} of length 3, then the corresponding pre-order traversing the root element 3, these three elements 2 and then the left subtree is the root node sequentially recursion.

2.2 Code

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
class Solution:
    # 返回构造的TreeNode根节点
    def reConstructBinaryTree(self, pre, tin):
        # write code here
        if len(pre) == 0:
            return None
        elif len(pre) == 1:
            return TreeNode(pre[0])
        else:
            tree = TreeNode(pre[0])
            rootindex = tin.index(pre[0])
            tree.left = self.reConstructBinaryTree(pre[1:rootindex+1], tin[:rootindex])
            tree.right = self.reConstructBinaryTree(pre[rootindex+1:], tin[rootindex+1:])
            return tree

3. Binary Basics

c ++ version
python version