replacement of character in column by values

["El Imam Hanan Attia Rizk" <hanan.elimam@umontreal.ca>]

Dear S Plus user
I am working in a big data, I have two data frames that  I did bind by rows 
(rbind)  and I gut these columns(A,B,C,D)
 
A    B    C    D
 
0     0    0     0
NA NA NA NA
NA NA NA NA
0      0    0      0
1      1    0      1
NA NA NA NA
1      1    0     1

I would like to go through each column and replace NA by 0 if it is between 2 
zeros, and if it is between two ones replace it by 1
 
I would really appreciate if any of you have the chance to give me some 
suggestions.

Hanan Elimam
Ph.D student, Pharmaceutical Sciences
Université de Montréal
Faculté de pharmacie
Pavillon Jean-Coutu, bureau 3173
2940 Chemin de la polytechnique
Montréal, H3T 1J4
 
tél.:  514-343-6111, poste 0388
FAX: 514-343-7073