Initial K.E, K = (1/2)mu². At the maximum height, the vertical component of velocity is zero, but the horizontal component remains $v_x = u\cos(45°) = u/\sqrt{2}$. The K.E at the top is (1/2)m(v_x)² = (1/2)m(u²/2) = (1/2) * (1/2)mu² = K/2.