CareerCup

Why did you tag this a C#? Did the interviewer require you write this in C#?

Any language/data structure is ok.

The easiest method for doing something like this is to use a native string.replace(" ", "%d") using a method that receives the character to be replaced and the type to replace it with to return [convert(" ", Type.HTML) returns "%d"]

A StringBuffer is another valid data structure to use, simply remove the found instance of the character to replace and then insert it's conversion

A character array would work for lower level languages but would require the overhead of having to manually shift characters down and resizing the array manually.