Solve the following recurrence relation using substi$-$

tution method $($forward or backward substitution$)$

$($a$)$ T $($n$)=$ T $($n $1)+$ n T $(0)=0$

$($b$)$ T $($n$)=2$T $($n $1)+2$n T $(0)=0$

$($c$)$ T $($n$)=2$T $($n $1)+1$ T $(0)=0$