You are a guitar player and you want to play a concert. Unfortunately, you don't have any good guitars left, so you need to buy some new guitars. You are given a String guitarNames, the i-th element of which is the name of the i-th guitar that is available for purchase.
You have a list of songs that you would like to play at the concert. Certain songs cannot be played with certain guitars because they will sound weird, so you might not be able to play the entire concert with just one guitar. You are given a String guitarSongs, where the j-th character of the i-th element is 'Y' if the j-th song can be played on the i-th guitar, and 'N' otherwise.
You want your concert to be as long as possible, so your main goal is to play as many of the songs as possible (you can only play each song at most once). You also want to save your money, so you want to buy the least number of guitars required to play that maximum number of songs. Return a String containing the names of the guitars you should buy in alphabetical order. If there are multiple possible return values, return the one among them that comes first lexicographically. A String s1 comes before String s2 lexicographically if s1[i] comes before s2[i] alphabetically, where i is the first position at which they differ.