add best_time_to_buy_and_sell_stock.cpp
add best_time_to_buy_and_sell_stock_II.cpp NOT accepted because it assume you can buy and sell at anytime.
add best_time_to_buy_and_sell_stock_II.cpp NOT accepted
add best_time_to_buy_and_sell_stock_II.cpp accepted
add best_time_to_buy_and_sell_stock_III.cpp