Revolution III

[h w b]=size(a);
for i=1:h*w
 c=[];
 while any(i)
  c=[c; i(1)];
  [v z]=ind2sub(h,i);
  t=i+[h*[z<w -(z>1)] v<h -(v>1)];
  i=t(a(t)==a(i)+1);
 end
 if nnz(c)>nnz(b)
  b=c;
 end
end