README.md

Binary Tree Diameter

Source: https://www.algoexpert.io/questions/binary-tree-diameter
Difficulty: Medium
Category: Binary Trees

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Write a function that takes in a Binary Tree and returns its diameter. The diameter of a binary tree is defined as the length of its longest path, even if that path doesn't pass through the root of the tree.

A path is a collection of connected nodes in a tree, where no node is connected to more than two other nodes. The length of a path is the number of edges between the path's first node and its last node.

Each BinaryTree node has an integer value, a left child node, and a right child node. Children nodes can either be BinaryTree nodes themselves or None / null.

Sample Input

tree =      1
          /   \
         3     2
       /   \ 
      7     4 
    /         \
   8           5
  /              \
 9                6

Sample Output

6 // 9 -> 8 -> 7 -> 3 -> 4 -> 5 -> 6
// There are 6 edges between the 
// first node and the last node 
// of this tree's longest path.

Hints

Hint 1

How can you use the height of a binary tree and the heights of its subtrees to calculate its diameter?

Hint 2

The length of the longest path that goes through the root of a binary tree is the sum of the heights of its left and right subtrees (left subtree height + right subtree height). The diameter of a binary tree can be calculated by taking the maximum of: 1) the maximum subtree diameter (max(left subtree diameter, right subtree diameter)); and 2) the length of the longest path that goes through the root (left subtree height + right subtree height).

Hint 3

Implement a variation of depth-first search that recursively keeps track of both the diameter and the height of a each subtree in the input binary tree. Follow Hint #2 to continuously compute these diameters.

Optimal Space & Time Complexity

Average case: when the tree is balanced O(n) time | O(h) space - where n is the number of nodes in the Binary Tree and h is the height of the Binary Tree