Maximum Depth of Binary Tree

Easy~15 min

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.

Each node has a val (integer value), a left pointer, and a right pointer. Leaf nodes point to null on both sides.

Examples

Example 1
Input: root = [3, 9, 20, null, null, 15, 7]
Output: 3
Explanation: The longest path is 3 -> 20 -> 15 (or 3 -> 20 -> 7), which has 3 nodes.
Example 2
Input: root = [1, null, 2]
Output: 2
Explanation: The longest path is 1 -> 2, which has 2 nodes.
Example 3
Input: root = []
Output: 0
Explanation: An empty tree has depth 0.

Constraints

  • The number of nodes in the tree is in the range [0, 10000]
  • -100 <= Node.val <= 100
  • Expected time complexity: O(n)
Code
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Output

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