Solving Programming Challenges

Answer 1:
Just the solution of the problem.
Answer 2:
Integer overflow.
Answer 3:
Test on the examples from the problem statement. Then make a few other small tests, solve them manually and check that your program outputs the correct answer. Generate a big input and launch your program to check that it works fast enough and doesn't consume too much memory. Test for corner cases: smallest allowed values and largest allowed values of all input parameters, equal numbers in the input, very long strings, etc. Then make a stress test. After all these tests passed, submit the solution.
Answer 4:
You generate valid input data as a part of the stress test implementation.
Answer 5:
If it is one of the first few tests, you will see the input data, the answer of your program and the correct answer. Otherwise, you will only see either that the answer of your program is wrong or that your program is too slow or that your program uses too much memory.