104. Maximum Depth of Binary Tree

https://leetcode.com/problems/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.

Example:

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

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

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Intuition
F(node) = 1 + Max( F(node.left), F(node.right))
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Time - O(n)
Space - O(h) - recursive call stack
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Related problems
111-minimum-depth-of-binary-tree
366-find-leaves-of-binary-tree
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