procedure build_next;begin    lena:=length(a);lenb:=length(b);    next[0]:=lenb;next[1]:=lenb-1;    for i:=1 to lenb-2 do if b[i]<>b[i+1] then     begin        next[1]:=i;break;    end;    k:=1;    for i:=2 to lenb-1 do     begin        p:=k+next[k]-1;L:=next[i-k];        if i+L<=p then next[i]:=L else        begin            j:=p-i+1;            if j<0 then j:=0;            while (i+j
     
      b[i] then    begin        ex[0]:=i;break;    end;    k:=0;    for i:=1 to lena-1 do     begin        p:=k+ex[k]-1;L:=next[i-k];        if i+L<=p then ex[i]:=L else        begin            j:=p-i+1;            if j<0 then j:=0;            while (i+j