With leapfrog you can use x + v dt/2 for the positions to calculate the energy. Or use this update rule:

    x += v*dt/2
    v += a(x)*dt
    x += v*dt/2

This gives you the x,v at the same time, yet still doesn't require storing the accelerations. The acceleration calculation can imperatively update v :)

In fact, if you call that update leapfrog(dt), then you can get a fourth order symplectic method like so:

    leapfrog(c1*dt)
    leapfrog(c2*dt)
    leapfrog(c1*dt)

Where c1,c2 are particular coefficients which you have to ask Yoshida.