  A search for a heavy resonance decaying into a top quark and antiquark
($\mathrm{t\bar{t}}$) pair is performed using proton-proton collisions at
$\sqrt{s} =$ 13 TeV. The search uses the data set collected with the CMS
detector in 2016, which corresponds to an integrated luminosity of 35.9
fb$^{-1}$. The analysis considers three exclusive final states and uses
reconstruction techniques that are optimized for top quarks with high Lorentz
boosts, which requires the use of nonisolated leptons and jet substructure
techniques. No significant excess of events relative to the expected yield from
standard model processes is observed. Upper limits on the production cross
section of heavy resonances decaying to a $\mathrm{t\bar{t}}$ pair are
calculated. Limits are derived for a leptophobic topcolor Z' resonance with
widths of 1, 10, and 30%, relative to the mass of the resonance, and exclude
masses up to 3.80, 5.25, and 6.65 TeV, respectively. Kaluza-Klein excitations
of the gluon in the Randall-Sundrum model are excluded up to 4.55 TeV. To date,
these are the most stringent limits on $\mathrm{t\bar{t}}$ resonances.
