Whiteboard interview, one-on-one.

The interviewer started the question without any constraints, and as I kept giving him solutions, he tightened the constraints.

(Detailed explanation in my answer.)

The whole question took about 10 minutes.

Describe a C function that performs the following:

Input: an integer. Output: If the integer is 4, the function returns 7. If the integer is 7, the function returns 4.

Constraints:

• The input is guaranteed to be either 4 or 7. No validation is needed.
• You are not allowed to use any conditional flow commands (if, switch, while, for, ternary operator, etc.).
• You are not allowed to use any external libraries or imports (like math, etc.).
• You are not allowed to use the modulo operator (%) or multiplication (*).