Binary search - suffix array

I am trying to combine the binary tree with the suffix array, but dont know if the suffix should be used as the key, or as the value. My initial thought would be to use the suffixes as keys, and the original index as values like so:

class SAS(object):

    def __init__(self, string):
        self.string_SA = Binary_tree.BSTree()
        for i in range(0, len(string)):
            self.string_SA.set(string[i:], i)

But what use is it to know this index?

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