root of a binary tree, return its maximum depth.
A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
Input: root = [3,9,20,null,null,15,7] Output: 3
Input: root = [1,null,2] Output: 2
# 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: Optional[TreeNode]) -> int: # It's easier to use our own function so we can avoid self. # cleaner too def calculate(root): # Basecase always first in recursive algos if root == None: return 0 # calculate left then right, order doesn't matter but this is a DFS search which is something we can say in an interview to sound smarter left = calculate(root.left) right = calculate(root.right) # add + 1 to the depth as we've gone down a level # we only care about the max, so calculate the max of both subtrees return 1 + max(left, right) return calculate(root)