1008.construct-binary-search-tree-from-preorder-traversal.py

#
# @lc app=leetcode id=1008 lang=python3
#
# [1008] Construct Binary Search Tree from Preorder Traversal
#
# https://leetcode.com/problems/construct-binary-search-tree-from-preorder-traversal/description/
#
# algorithms
# Medium (81.12%)
# Likes:    5010
# Dislikes: 67
# Total Accepted:    285.8K
# Total Submissions: 352.4K
# Testcase Example:  '[8,5,1,7,10,12]'
#
# Given an array of integers preorder, which represents the preorder traversal
# of a BST (i.e., binary search tree), construct the tree and return its root.
# 
# It is guaranteed that there is always possible to find a binary search tree
# with the given requirements for the given test cases.
# 
# A binary search tree is a binary tree where for every node, any descendant of
# Node.left has a value strictly less than Node.val, and any descendant of
# Node.right has a value strictly greater than Node.val.
# 
# A preorder traversal of a binary tree displays the value of the node first,
# then traverses Node.left, then traverses Node.right.
# 
# 
# Example 1:
# 
# 
# Input: preorder = [8,5,1,7,10,12]
# Output: [8,5,10,1,7,null,12]
# 
# 
# Example 2:
# 
# 
# Input: preorder = [1,3]
# Output: [1,null,3]
# 
# 
# 
# Constraints:
# 
# 
# 1 <= preorder.length <= 100
# 1 <= preorder[i] <= 1000
# All the values of preorder are unique.
# 
# 
#

# @lc code=start
# 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
class Solution:
    def bstFromPreorder(self, preorder: List[int]) -> Optional[TreeNode]:
        if not preorder:
            return None
        root = TreeNode(preorder[0])
        for i in range(1, len(preorder)):
            self.insert(root, preorder[i])
        return root
    
    def insert(self, root: TreeNode, val: int) -> None:
        if val < root.val:
            if not root.left:
                root.left = TreeNode(val)
            else:
                self.insert(root.left, val)
        else:
            if not root.right:
                root.right = TreeNode(val)
            else:
                self.insert(root.right, val)
        
# @lc code=end