Do you know how i can solve the problem use a python code:

Task is to calculate how many subsequences of the given string contain the characters t$,$ i$,$ r$,$ and a in any order.

The algorithm's time complexity should be O$($n$).$

Code returns the desired number of subsequences.

python code

def count$($s$)$:

# TODO

if $\_\_$name$\_\_=="\_\_$main$\_\_"$:

print$($count$("$tira$"))$ # $1$

print$($count$("$ritari$"))$ # $6$

print$($count$("$tiratiratira$"))$ # $45$

print$($count$("$xaxrxixtx$"))$ # $4$

Explanation: For example, in the string "ritari," there are six desired subsequences: itar, itari, rita, ritar, ritari, and tari.