Is Longest Univalue Path easy or hard?

Longest Univalue Path is considered a Medium difficulty problem on LeetCode. The challenge lies in correctly combining left and right subtree paths while ensuring only nodes with equal values extend the path.

Longest Univalue Path Python/Java solution

Python and Java implementations typically use recursive DFS. Each recursive call returns the longest same-value path extending from the current node, while a global variable stores the maximum path found anywhere in the tree.

How to solve Longest Univalue Path in O(n)?

Use a postorder DFS traversal. For each node, compute the longest path from the left and right child that continues the same value. If a child matches the node's value, extend that path by one edge. Update a global maximum with the sum of both sides and return the longer side to the parent.

What is the best approach for Longest Univalue Path?

Depth First Search with a global maximum is the most effective approach. A postorder traversal calculates the longest same-value path from each node's left and right children, then updates a global answer using leftPath + rightPath. This method visits every node once, giving O(n) time complexity.

Is Longest Univalue Path asked at Google/Amazon/Meta?

Tree DFS problems similar to Longest Univalue Path frequently appear in interviews at companies like Amazon, Google, and Meta. Interviewers use variations of tree path problems to test recursion, postorder traversal, and the ability to combine results from subtrees.

What data structure is used in Longest Univalue Path?

The problem uses a binary tree as the primary data structure. The optimal algorithm applies Depth-First Search recursion to traverse the tree and compute path lengths from child nodes back to their parent.

What is the time complexity of Longest Univalue Path?

The optimal solution runs in O(n) time because each node in the binary tree is processed exactly once during DFS traversal. The recursion stack uses O(h) space, where h is the height of the tree. In the worst case of a skewed tree, space can reach O(n).

Description Solution Video 8:42

687. Longest Univalue Path

Medium44.3% acceptance7 companies

Given the root of a binary tree, return the length of the longest path, where each node in the path has the same value. This path may or may not pass through the root.

The length of the path between two nodes is represented by the number of edges between them.

Example 1:

Input: root = [5,4,5,1,1,null,5]
Output: 2
Explanation: The shown image shows that the longest path of the same value (i.e. 5).

Example 2:

Input: root = [1,4,5,4,4,null,5]
Output: 2
Explanation: The shown image shows that the longest path of the same value (i.e. 4).

Constraints:

The number of nodes in the tree is in the range [0, 10⁴].
-1000 <= Node.val <= 1000
The depth of the tree will not exceed 1000.

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687. Longest Univalue Path

Asked at 7 companies

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