Create Binary Tree From Descriptions — LeetCode 2196 Python Solution

The 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.

Example

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.

Python solution

# 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]:
        nodes = defaultdict(TreeNode)
        children = set()
        for parent, child, isLeft in descriptions:
            if parent not in nodes:
                nodes[parent] = TreeNode(parent)
            if child not in nodes:
                nodes[child] = TreeNode(child)
            children.add(child)
            if isLeft:
                nodes[parent].left = nodes[child]
            else:
                nodes[parent].right = nodes[child]
        root = (set(nodes.keys()) - children).pop()
        return nodes[root]

Complexity

Pattern: Tree Traversal

Choose the order — preorder, inorder, postorder, level — and the problem solves itself. LeetCode 2196. Create Binary Tree From Descriptions is filed here because LeetCode tags it Tree and Binary Tree, which is the vocabulary this hub collects.

The tree traversal guide has the Python template for the pattern and the 225 LeetCode problems that use it.

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