105. Construct Binary Tree from Preorder and Inorder Traversal

Description:

Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

 

 

 

 

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right

'''
[1] divide and set)

idea::
set root using preorder, divide left-subtree and right-subtree using inorder

code::
(below)

-T/C: O(n)
-S/C: O(n) # for making tree
'''
class Solution:
    def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
        return self.builder(preorder,inorder)
        
    def builder(self, preorder,inorder):
        # exception
        if len(preorder)<=0:
            return None
        
        # set root
        root=TreeNode(preorder[0])
        
        # find root' idx
        root_idx=0
        for i,num in enumerate(inorder):
            if num==root.val:
                root_idx=i
                break
        
        # divide
        left_size=root_idx
        
        # set left and right
        root.left=self.builder(preorder[1:left_size+1],inorder[0:left_size])
        root.right=self.builder(preorder[1+left_size:],inorder[root_idx+1:])
        return root