how can i make code for implicit method for CFD application?

in fact, I have already created code for implicit method, but i have alots of issue on it, so please im looking for help and support for this point quick. My code

clear all
clc
format short
wallLegnth = 1;
dy = 0.05;
toataltime = 0.5;
dt = 0.01;
alpha = 0.1;
dis=0:dy:1;
yn = (wallLegnth/dy)+1;
tn = (toataltime/dt)+1;
alpha=0.1;
z=alpha*dt/(dy^2);
a=-z;
b=(2*z+1);
c=-z;
TRI=full(gallery('tridiag',yn-2,a,b,c));
TRII=inv(TRI);

% Initial condition
for j=2:1:yn-1;
   LM(1,j)=100;
end

%Boundary condition
for j=1:1:tn;
    LM(j,1)=300;
    LM(j,yn)=300;
end

%solveing 

for i=1:1:yn;
    for j=1;tn-1;

    if i==1;
        D(i,j)= LM(i+1,j)+a*LM(i,j+1);

          elseif 1<i<tn-2
       D(i+1,j)=LM(i+1,j);

        if i==tn-2
        D(i-1,j)= LM(i+1,j)+a*LM(i+2,j+1);
        end
    end
      end
  end

for i=1:1:tn;
    for j=2:1:yn-1;
        LM(i,j)=D'*TRII;
    end 
end

Answer 1

[Susmit Kumar Mishra]

ig this LM(i,j)=D'*TRII;

should be LM(i,j)=TRII*D';