Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root. https://leetcode.com/explore/challenge/card/30-day-leetcoding-challenge/529/week-2/3293/

BinaryTreeDiameterCalculatorWithRecursion.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */

import java.util.*;

class BinaryTreeDiameterCalculatorWithRecursion {
    
    public int diameterOfBinaryTree(TreeNode root) {
        SolutionFinder finder = new SolutionFinder();
        findDepth(root, finder);
        return finder.value;
    }
    
    /**
     * 
     */
    private int findDepth(TreeNode currentRoot, SolutionFinder finder){
        if(currentRoot==null) {
            return 0;
        }
        
        // Calculate the depth of a node in the usual way 
        // max(depth of node.left, depth of node.right) + 1
        int leftDepth = findDepth(currentRoot.left, finder);
        int rightDepth = findDepth(currentRoot.right, finder);
        int currentDepth = 1 + Math.max(leftDepth,rightDepth);
        
        // Check if current diameter is the maximum and save it
        int currentDiameter = leftDepth + rightDepth;
        finder.suggestDiameter(currentDiameter);
        
        return currentDepth;
    }
    
    private class SolutionFinder {
        private int value = 0;
        private void suggestDiameter(int currentDiameter){
            if(this.value < currentDiameter) {
                this.value = currentDiameter;
            }
        }
    }
}