In general, using strings does not seriously affect Big-O notation. Of course, it depends on what you're doing with it.
Recall that a string can be considered an array of characters. Thus, if you were accessing each character of a string, this would be the same as accessing each member of an integer array (a.k.a. O(n)). Also, using a string method (such as stringVariable.find(char OR string)) will have a certain run-time - .find() probably has O(n) (since it will perform a linear search through the string), and .substr() probably has O(1) (since it will take a constant time to get the substring). Of course, these are just my guesses, so I'm not completely sure.
Generally, treat strings as character arrays for purposes of determining Big-O notation,
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