Is Find All The Lonely Nodes easy or hard?

Find All The Lonely Nodes is classified as an Easy problem. The main challenge is recognizing that a simple tree traversal with a parent-child check is sufficient, making it a good practice problem for binary tree fundamentals.

Find All The Lonely Nodes Python/Java solution

A typical Python or Java solution performs a DFS traversal starting from the root. At each node, check if only one child exists and append that child’s value to a result list. Continue recursively for both children until the entire tree is explored.

How to solve Find All The Lonely Nodes in O(n)?

Traverse the binary tree using DFS or BFS. For every node, check if exactly one child exists. If the left child exists but the right does not, record the left node; if the right exists but the left does not, record the right node. Continue traversal until all nodes are processed.

What is the best approach for Find All The Lonely Nodes?

Depth-First Search (DFS) is the most common approach. Traverse the binary tree and check each node’s children. If a node has exactly one child, add that child’s value to the result. DFS visits every node once, giving O(n) time complexity with O(h) recursion stack space.

Is Find All The Lonely Nodes asked at Google/Amazon/Meta?

This problem represents a common binary tree traversal pattern often seen in technical interviews. Variations of parent-child relationship checks appear in interviews at large tech companies including Google, Amazon, and Meta, especially in easy-to-medium tree rounds.

What data structure is used in Find All The Lonely Nodes?

The primary data structure is a binary tree. The algorithm uses either recursion (DFS) with the call stack or an explicit queue for BFS traversal to visit each node and inspect its children.

What is the time complexity of Find All The Lonely Nodes?

The time complexity is O(n), where n is the number of nodes in the binary tree. Each node is visited exactly once during the traversal, and checking whether a node has one child takes constant time.

Description Hints 1 Solution Video 6:23

1469. Find All The Lonely Nodes

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In a binary tree, a lonely node is a node that is the only child of its parent node. The root of the tree is not lonely because it does not have a parent node.

Given the root of a binary tree, return an array containing the values of all lonely nodes in the tree. Return the list in any order.

Example 1:

Input: root = [1,2,3,null,4]
Output: [4]
Explanation: Light blue node is the only lonely node.
Node 1 is the root and is not lonely.
Nodes 2 and 3 have the same parent and are not lonely.

Example 2:

Input: root = [7,1,4,6,null,5,3,null,null,null,null,null,2]
Output: [6,2]
Explanation: Light blue nodes are lonely nodes.
Please remember that order doesn't matter, [2,6] is also an acceptable answer.

Example 3:


Input: root = [11,99,88,77,null,null,66,55,null,null,44,33,null,null,22]
Output: [77,55,33,66,44,22]
Explanation: Nodes 99 and 88 share the same parent. Node 11 is the root.
All other nodes are lonely.

Constraints:

The number of nodes in the tree is in the range [1, 1000].
1 <= Node.val <= 10⁶

Asked at 1 company

Microsoft

Topics

Hints open one at a time

Hint 1

Do a simple tree traversal, try to check if the current node is lonely or not.

6:23

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