function xstar = invinterp(x,y,ystar)

n = length(x);
xstar = [];
numxstar = 0;
for i = 1:n-1
    if( (y(i)-ystar)*(y(i+1)-ystar) < 0)
        numxstar = numxstar + 1;
        xs = x(i-1:i+2);
        ys = y(i-1:i+2);
        S = spline(ys,xs);
        xstar(numxstar) = ppval(S,ystar);
    end
end