function XK=ditfft(xn,N)
%it is a function of DIT_FFT based 2 .
%the argument 'xn'is the sample sequence input.
%the argument 'N'is the length of the sample sequence input.
%------------------add zeros----------------------------%
if length(xn)<N    
    xn=[xn,zeros(1,N-length(xn))];    
end
%--------------bit reversal（Rader Algorithm）-----------%
j=0;
for t=0:N-1
    if t<j
       xn(j+1)=xn(j+1)+xn(t+1);
       xn(t+1)=xn(j+1)-xn(t+1); 
       xn(j+1)=xn(j+1)-xn(t+1);
     end
         k=N/2;             
      while k<=j             
         j=j-k;k=k/2;
      end
        j=j+k;
end    
 %---------------FFT MAIN FUNCTION---------------------%
 le=1;
 L=log2(N);
 for m=1:L 
     lei=le;
     le=le*2;
     u=exp(0);
     w=exp(-pi*i/lei);
     for j=0:lei-1 
        for k=j:le:N-1    
             p=k+lei;
             r=xn(p+1)*u;
             xn(p+1)=xn(k+1)-r;
             xn(k+1)=xn(k+1)+r;
         end
         u=u*w;
     end 
 end
 XK=xn;