T(n) = T(n-1) + n ..............(0)
T(n-1)= T((n-1)-1) + n-1
=T(n-2)+n-1 ..............(1)
T(n-2)= T((n-2)-1) + n-2
=T(n-3)+n-2 ..............(2)
Now substituting (2) in (1)
T(n-1)= (T(n-3)+n-2)+n-1
= T(n-3)+2n-3 ..............(4)
Now substituting (4) in (0)
T(n) = (T(n-3)+2n-3 )+ n
=T(n-3)+3n-3
...
...
T(n)=T(n-k)+kn-k
put k=n-1 in above equation
T(n)=T(n-n+1) + (n-1)n - (n-1)
T(n) = T(1) + n^2 - n -n + 1
∴ The solution is O(n^2)