Step 1: Simple Calculator Create a simple String calculator with a single method:

class StringCalculator { int Add(string numbers); } The method can take 1 or 2 comma-separated numbers, and will return their sum.

The method returns 0 when passed the empty string.

Example:

Add("") // 0 Add("4") // 4 Add("1,2") // 3 Start with the simplest test case of an empty string and move to 1 and two numbers.

Step 2: Arbitrary number size Allow the Add method to handle an unknown amount of numbers.

Example:

Add("1,2,3,4,5,6,7,8,9") // 45 Step 3: Newline separator Allow the Add method to recognise newlines as well as commas as separators. The two separator types can be used interchangeably.

NB: Focus on the happy path - since this is not production code, it's fine if the code crashes if it's given invalid input (e.g. "1,\n2").

Example:

Add("1\n2,3") // 6 Step 4: Custom separators Optionally support custom separators. To change separator, the beginning of the string will contain a separate line that looks like this: “//\n”

Example:

Add("//;\n1;2") // 3 Note that all existing scenarios should still be supported.

Step 5: Disallow negatives Calling Add with a negative number will throw an exception negatives not allowed, and the negative that was passed.

If there are multiple negatives, show all of them in the exception message.

Example:

Add("1,-2,-3") // error: negatives not allowed: -2 -3 Step 6: Ignore numbers bigger than 1000 Numbers bigger than 1000 should be ignored.

Example:

Add("1001, 2") // 2 Step 7: Arbitrary-length separators Separators can be of any length if surrounded by square brackets.

Example:

Add("//[]\n12***3") // 6 Step 8: Multiple single-length separators Allow multiple single-character separators like this: “//[delim1][delim2]\n”

Example:

Add("//[][%]\n12%3") // 6 Step 9: Multiple longer-length separators Handle multiple separators with any character length.

Example:

Add("//[foo][bar]\n1foo2bar3") // 6 Credit: Roy Osherove