indexToNumber to map indices to numbers.numberToIndices to map numbers to a TreeSet of indices.change method:
a. If the index already has a number, remove the index from the TreeSet of the old number.
b. Update the indexToNumber map with the new number for the given index.
c. Add the index to the TreeSet of the new number in numberToIndices.find method:
a. Check if the number exists in numberToIndices.
b. If it does, return the first (smallest) index in the TreeSet.
c. If not, return -1.