June 7Medium

Create binary tree from descriptions

Problem

You are given a 2D integer array descriptions where descriptions[i] = [parenti, childi, isLefti] indicates that parenti is the parent of childi in a binary tree of unique values.

Furthermore, If isLefti == 1, then childi is the left child of parenti. If isLefti == 0, then childi is the right child of parenti. Construct the binary tree described by descriptions and return its root.

The test cases will be generated such that the binary tree is valid.

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

Solution

class Solution:
    def createBinaryTree(self, descriptions: List[List[int]]) -> Optional[TreeNode]:
        parents, children = set(), set()
        nodes = {}

        def get_or_create( value):
            if value in nodes:
                return nodes[value]
            
            node = TreeNode(value)
            nodes[value] = node
            return node

        for parent, child, isLeft in descriptions:
            p_node = get_or_create(parent)
            if isLeft:
                p_node.left = get_or_create(child)
            else:
                p_node.right = get_or_create(child)
            parents.add(parent)
            children.add(child)

        for parent in list(parents):
            if parent not in children:
                return nodes[parent]