/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
const maxDepth = (root) => {
  let depth = 0;

  const dfs = (currNode, tmpDepth) => {
    if (null !== currNode) {
      tmpDepth++;
      dfs(currNode.left, tmpDepth);
      dfs(currNode.right, tmpDepth);
    } else {
      depth = Math.max(depth, tmpDepth);
      return;
    }
  }

  dfs(root, 0);

  return depth;
};