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2196. Create Binary Tree From Descriptions

Difficulty Topics

Description

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.

 

Example 1:

Input: descriptions = [[20,15,1],[20,17,0],[50,20,1],[50,80,0],[80,19,1]]
Output: [50,20,80,15,17,19]
Explanation: The root node is the node with value 50 since it has no parent.
The resulting binary tree is shown in the diagram.

Example 2:

Input: descriptions = [[1,2,1],[2,3,0],[3,4,1]]
Output: [1,2,null,null,3,4]
Explanation: The root node is the node with value 1 since it has no parent.
The resulting binary tree is shown in the diagram.

 

Constraints:

  • 1 <= descriptions.length <= 104
  • descriptions[i].length == 3
  • 1 <= parenti, childi <= 105
  • 0 <= isLefti <= 1
  • The binary tree described by descriptions is valid.

Solution

create-binary-tree-from-descriptions.py
# 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 createBinaryTree(self, descriptions: List[List[int]]) -> Optional[TreeNode]:
        deg = collections.defaultdict(int)
        graph = collections.defaultdict(list)
        mp = {}

        for parent, child, isLeft in descriptions:
            deg[child] += 1

            graph[parent].append((child, isLeft))

        def go(parent):
            node = TreeNode(parent)

            for child, isLeft in graph[parent]:
                if isLeft == 1:
                    node.left = go(child)
                else:
                    node.right = go(child)

            return node

        for parent, _, __ in descriptions:
            if deg[parent] == 0:
                return go(parent)