package main

type TreeNode struct {
	Val   int
	Left  *TreeNode
	Right *TreeNode
}

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func maxDepth(root *TreeNode) int {
	depth := 0
	queue := []*TreeNode{root}
	if root == nil {
		return 0
	}
	for len(queue) != 0 {
		current := queue
		queue = queue[len(queue):]
		for _, node := range current {
			if node.Left != nil {
				queue = append(queue, node.Left)
			}
			if node.Right != nil {
				queue = append(queue, node.Right)
			}
		}
		depth++
	}
	return depth
}