LeetCode 104. Maximum Depth of Binary Tree

Example:

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

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

	class Solution {
	public int maxDepth(TreeNode root) {
	// corner case
	if (root == null) {
	return 0;
	}
	// call the childrens recursively
	return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
	}
	}

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