104. Maximum Depth of Binary Tree

1. Description

Given the 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.

2. Example

Example 1

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2

Input: root = [1,null,2]
Output: 2

3. Constraints

• The number of nodes in the tree is in the range [0, $10^4$].
• -100 <= Node.val <= 100

4. Solutions

Recursive

n is the number of nodes in root
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    int maxDepth(TreeNode *root) {
        return root == nullptr ? 0 : 1 + max(maxDepth(root->left), maxDepth(root->right));
    }
};