binarytreediameter.js

class TreeNode {
  constructor(val) {
    this.val = val;
    this.left = null;
    this.right = null;
  }
}

function diameterOfBinaryTree(root) {
  let diameter = 0;

  function depth(node) {
    if (!node) {
      return 0;
    }

    const left = depth(node.left);
    const right = depth(node.right);

    diameter = Math.max(diameter, left + right);

    return Math.max(left, right) + 1;
  }

  depth(root);

  return diameter;
}