Recently, isotope shifts of (72,74-96Kr) and Sr-77-100 have been measured at the ISOLDE/CERN mass separator facility by collinear laser spectroscopy. The deduced changes in mean-square charge radii reveal sharp transitions in nuclear shape from spherical near the magic neutron number N = 50 towards strongly deformed for both the neutron-deficient and neutron-rich isotopes far from stability. The mean-square charge radii of the neutron-deficient isotopes exhibit a sign change of the odd-even staggering (OES), i.e. below the neutron number N = 46 the radius is systematically larger for the odd-N nuclei than for their even-N neighbours. This is in contrast to the situation of normal OES which is observed for the heavier isotopes. The inversion of the OES is interpreted as an effect of polarization, triggered by the addition of an unpaired neutron and driving the soft even-even core into stable strong deformation.