题目来源：104. 二叉树的最大深度

给定一个二叉树，找出其最大深度。

二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。

说明: 叶子节点是指没有子节点的节点。

示例：
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 。

/**
 * 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}
 */
var maxDepth = function(root) {
  let h = 0
  if(!root){
    return h
  }
  let stack = []
  stack.push([root])
  while(stack.length){
    let curLevel = stack.shift()
    let len = curLevel.length
    if(len>0){
      h += 1
    }
    let level = []
    for(let i = 0;i<len;i++){
      let node = curLevel[i]
      if(node.left){
        level.push(node.left)
      }
      if(node.right){
        level.push(node.right)
      }
    }
    if(level.length>0){
      stack.push(level)
    }
  }
  return h
};

/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxDepth = function(root) {
  let h = 0
  if(!root){
    return h
  }
  let left = maxDepth(root.left)
  let right = maxDepth(root.right)
  return Math.max(left, right) + 1
};

 

Python3

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: TreeNode) -> int:
      res = 0
      if not root:
        return res
      stack = list()
      stack.append([root])
      while stack:
        curLevel = stack.pop(0)
        if curLevel:
          res += 1
        level = list()
        for node in curLevel:
          if node.left:
            level.append(node.left)
          if node.right:
            level.append(node.right)

        if level:
          stack.append(level)

      return res        

 

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: TreeNode) -> int:
      if not root:
        return 0
      else:
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)
        return max(left, right) + 1