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Compute subtree sums with tree DFS | Netflix Coding Question

Quick Overview

This question evaluates proficiency in tree traversal and aggregation techniques, specifically computing subtree sums using depth-first search while handling traversal state and constraints such as negative values and large n.

Compute subtree sums with tree DFS

Company: Netflix

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: hard

Interview Round: Onsite

You are given an undirected tree with `n` nodes labeled `1..n` and `n-1` edges. The tree is rooted at node `1`. Each node `i` has an integer value `val[i]`. Return an array `subtreeSum[1..n]` where `subtreeSum[u]` is the sum of `val[x]` over all nodes `x` in the subtree of `u` (including `u`). Constraints: - `1 <= n <= 2e5` - Values can be negative. - The input is guaranteed to be a tree. Implement an `O(n)` solution using DFS.

Quick Answer: This question evaluates proficiency in tree traversal and aggregation techniques, specifically computing subtree sums using depth-first search while handling traversal state and constraints such as negative values and large n.

You are given an undirected tree with n nodes labeled from 1 to n, rooted at node 1. Each node i has an integer value val[i]. Return a list of length n where the element at index i-1 is the sum of values of all nodes in the subtree of node i, including node i itself. Your solution should run in O(n) time.

Constraints

1 <= n <= 2 * 10^5
len(edges) == n - 1
-10^9 <= val[i] <= 10^9
The input graph is guaranteed to be a tree
Values may be negative

Examples

Input: (5, [(1, 2), (1, 3), (3, 4), (3, 5)], [4, 2, 1, 3, 5])

Expected Output: [15, 2, 9, 3, 5]

Explanation: Node 3 has subtree {3,4,5} with sum 1+3+5=9. Node 1 includes all nodes, so its sum is 15.

Input: (4, [(1, 2), (2, 3), (2, 4)], [1, -2, 4, -1])

Expected Output: [2, 1, 4, -1]

Explanation: Node 2 has subtree sum -2+4+(-1)=1, and node 1 has 1+1=2.

Input: (1, [], [-7])

Expected Output: [-7]

Explanation: A single node's subtree consists only of itself.

Input: (6, [(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)], [5, 0, -2, 1, 1, 1])

Expected Output: [6, 1, 1, 3, 2, 1]

Explanation: This is a chain. Compute from the end upward: node 6=1, node 5=2, node 4=3, node 3=1, node 2=1, node 1=6.

Input: (4, [(1, 2), (1, 3), (1, 4)], [0, 0, 0, 0])

Expected Output: [0, 0, 0, 0]

Explanation: All node values are zero, so every subtree sum is zero.

Hints

Build an adjacency list for the tree, then run DFS from node 1 while tracking each node's parent.
A node's subtree sum depends on its children, so compute results in postorder: children first, then parent.

Last updated: Apr 19, 2026

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