class Solution {

private static int treeDiameter = 0;

public int diameterOfBinaryTree(TreeNode root) {

calculateHeight(root);

return treeDiameter-1;

}

private static int calculateHeight(TreeNode currentNode) {

if (currentNode == null)

return 0;

int leftTreeHeight = calculateHeight(currentNode.left);

int rightTreeHeight = calculateHeight(currentNode.right);

// if the current node doesn't have a left or right subtree, we can't have

// a path passing through it, since we need a leaf node on each side

if (leftTreeHeight != 0 && rightTreeHeight != 0) {

// diameter at the current node will be equal to the height of left subtree +

// the height of right sub-trees + '1' for the current node

int diameter = leftTreeHeight + rightTreeHeight + 1;

// update the global tree diameter

treeDiameter = Math.max(treeDiameter, diameter);

}

// height of the current node will be equal to the maximum of the heights of

// left or right subtrees plus '1' for the current node

return Math.max(leftTreeHeight, rightTreeHeight) + 1;

}

}