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\begin{document}
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\begin{table}[h]
	\begin{tabular}{llr}
		\toprule
		Structure & \multicolumn{2}{l}{Access time complexity} \\ \cmidrule(r){2-3}
		& Average & Worst \\
		\midrule
		Stack & $\mathcal{O}(n)$ & $\mathcal{O}(n)$ \\
		Binary tree & $\mathcal{O}(log(n))$ & $\mathcal{O}(n)$ \\
		AVL tree & $\mathcal{O}(log(n))$ & $\mathcal{O}(log(n))$ \\
		\bottomrule
	\end{tabular}
\end{table}

\end{document}
