104. Maximum Depth of Binary Tree


Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.


Given binary tree [3,9,20,null,null,15,7],

3 / \ 9 20 / \ 15 7

return its depth = 3.

Thought Process

    Can solve this question recursively.
  • find the max d for the left tree recursively
  • find the max d for the right tree recursively
  • return the larger d

White Board

Below is the white board:


# Definition for a binary tree node.
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        if root is None:
            return 0
        left_d = self.maxDepth(root.left)
        right_d = self.maxDepth(root.right)

        return max(left_d, right_d) + 1


  • Time complexity is O(n)


Maybe have an iterative solution...