Is Most Frequent Subtree Sum easy or hard?

Most Frequent Subtree Sum is rated Medium difficulty. The recursion itself is straightforward, but the challenge comes from correctly computing subtree sums using post-order traversal and tracking frequencies efficiently.

Most Frequent Subtree Sum Python/Java solution

Python and Java implementations typically use recursive DFS with a hash map or dictionary to track sum frequencies. Python uses a dictionary or collections.Counter, while Java uses HashMap . Both achieve O(n) time and O(n) space complexity.

How to solve Most Frequent Subtree Sum in O(n)?

Perform a post-order depth-first traversal. For each node, compute the subtree sum using the sums of its left and right children. Store the frequency of each sum in a hash map and track the maximum frequency while traversing. After DFS completes, return all sums that match the highest frequency.

What is the best approach for Most Frequent Subtree Sum?

The best approach uses post-order DFS with a hash map to track frequencies of subtree sums. Each node computes its subtree sum after visiting its children, and the sum is recorded in the map. This processes the tree in a single traversal with O(n) time and O(n) space complexity.

Is Most Frequent Subtree Sum asked at Google/Amazon/Meta?

Binary tree DFS problems with subtree computations frequently appear in interviews at companies like Amazon, Meta, and Google. Variants involving subtree sums, subtree properties, or frequency counting are common because they test recursion, tree traversal, and hash map usage.

What data structure is used in Most Frequent Subtree Sum?

The solution relies on a hash table (hash map) to store the frequency of each subtree sum. The algorithm also uses recursion or an explicit stack for depth-first traversal of the binary tree.

What is the time complexity of Most Frequent Subtree Sum?

The optimal solution runs in O(n) time where n is the number of nodes in the binary tree. Each node is visited exactly once during the DFS traversal, and hash map updates occur in constant time on average.

Description Solution Video 9:51

508. Most Frequent Subtree Sum

Medium69.6% acceptance3 companies

Given the root of a binary tree, return the most frequent subtree sum. If there is a tie, return all the values with the highest frequency in any order.

The subtree sum of a node is defined as the sum of all the node values formed by the subtree rooted at that node (including the node itself).

Example 1: