104_Maximum_Depth_of_Binary_Tree.java

class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int leftHeight = maxDepth(root.left);
        int rightHeight = maxDepth(root.right);

        return Math.max(leftHeight, rightHeight) + 1;
    }
}