给定二叉树 [3,9,20,null,null,15,7]，

    3
   / \
  9  20
    /  \
   15   7
返回它的最大深度 3 。

let maxDepth = function (root) {
  let max = 0
  let helper = (node, depth) => {
    if (!node) return
    max = Math.max(max, depth)
    if (node.left) {
      helper(node.left, depth + 1)
    }
    if (node.right) {
      helper(node.right, depth + 1)
    }
  }
  helper(root, 1)
  return max
}

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
let maxDepth = function (root) {
  if (!root) return 0
  let max = 0
  let queue = [root]

  while (queue.length) {
    max += 1
    let len = queue.length
    while (len--) {
      let node = queue.shift()
      if (node.left) {
        queue.push(node.left)
      }
      if (node.right) {
        queue.push(node.right)
      }
    }
  }

  return max
}

let maxDepth = function(root) {
  if (!root) return 0
  return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1
};