Currency arbitrage detection

A trading desk has a matrix rates where rates[i][j] is how many units of currency j you get for one unit of currency i (rates[i][i] = 1, and rates[i][j] <= 0 means no direct conversion). An arbitrage opportunity exists if you can start with one unit of some currency, convert through a sequence of currencies, and return to the start with strictly more than one unit. Return True if any such opportunity exists, otherwise False. There are at most 50 currencies.