The below is the C# implementation for the a function for given parameters int a, int b - both represent characters 'A' and 'B' respectively, where the function should return a string containing both characters 'A' and 'B' occurring a times and b times respectively but neither 'A' nor 'B' repeating consecutively for more than 2 times. Both values for a and b are given in a way that a string can be build using those numbers - so eg: Foo(0,3) or Foo(1,7) shall not be invoked.


Foo(3,3) returns "BBAABA" or "AABBAB"

Foo(4,1) returns "AABAA"

Foo(3,5) returns "BAABBABB" or "BBAABBAB"


static string Foo(int a, int b)
    int total = a + b;
    StringBuilder sb = new StringBuilder();
    char charToPrint = a > b ? 'A' : 'B';
    int flag = 0;

    for(int x =0; x< total; x++)
        if(flag == 2)
            flag = 0;
            charToPrint = SwapChar(charToPrint);
        if(charToPrint == 'A' && a == 0 || charToPrint == 'B' && b == 0)
            charToPrint = SwapChar(charToPrint);

        if (charToPrint == 'A')

    return sb.ToString();

static char SwapChar(char thisChar)
    return thisChar == 'A' ? 'B' : 'A';

This is working but I would like to receive some feedback on this code.

  • \$\begingroup\$ It looks like you've forgotten to include your SwapChar method. What is the code supposed to do when it is impossible to create a string with the requirements? E.g. Foo(0, 3) is impossible but your code returns "BBB" which violates the last rule. \$\endgroup\$ – RobH Aug 5 '20 at 15:16
  • \$\begingroup\$ @RobH added the SwapChar method. Btw, I just edited the question to include the Foo(0,3) shall be called. \$\endgroup\$ – Isham Mohamed Aug 5 '20 at 15:24

Your algorithm has a flaw - it's too greedy. Take this example:

Foo(6, 2) // "AABBAAAA"

Alas, it should return "AABAABAA".

In terms of a review, your method shouldn't be called Foo - give it a descriptive name.

Great use of StringBuilder. You could initialize the length to a+b in the constructor as you know what size your finished string will be.

Expression bodied members can greatly improve readability in my opinion:

static char SwapChar(char c) => 
    c == 'A' ? 'B' : 'A';

Try to avoid variables like flag. I had to read the whole loop before I knew what that variable meant. I'd call it something like repeatCount.

  • \$\begingroup\$ Thanks for the feedback and indicating the flaw \$\endgroup\$ – Isham Mohamed Aug 6 '20 at 2:54

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