assuming string is "abcdefghijklmn ... "

essentially becomes

T(N) = "a" + T(N-1) +

"ab" + T(N-2) +

"abc" + T(N-3) +

...

...

there will be N constant terms, so

T(N)

= N + T(N-1) + T(N-2) + T(N-3) + ...

= N + N + N ...

= N^2

is that N^2 runtime?