text: /**
   * Definition for a binary tree node.
   * const TreeNode = (val, left, right) => {
   *     this.val = (val===undefined ? 0 : val)
   *     this.left = (left===undefined ? null : left)
   *     this.right = (right===undefined ? null : right)
   * }
   */
  /**
   * Find the maximum depth to a leaf node in a binary tree.
   * @param {TreeNode} root
   * @return {number}
   */
   const maxDepth = root => {
      // Edge case: No root node.
      if (!root) return 0
      
      /* Recursively call this method on the .left and .right nodes, incrementing by one each time.
         Return the maximum value when comparing the recursive result of .left and .right */
      return Math.max(1 + maxDepth(root.left),
                      1 + maxDepth(root.right))  
  }