Questions:\n* Valid parentheses in a string, about possible string permutations.\n* Simulate rain drops on a 1 meter wide walk way. Each rain drop is going to cover 1 centimeter of the walkway. Assume that a rain drop randomly falls from any position within the meter and falls straight down. Find out how many rain drops it takes to wet the whole walkway.

Questions:\n Valid parentheses in a string, about possible string permutations.\n Simulate rain drops on a 1 meter wide walk way. Each rain drop is going to cov