Implement a SnapshotArray that supports the following interface: SnapshotArray(int length) initializes an array-like data structure with the given length. Initially, each element equals 0. void set(index, val) sets the element at the given index to be equal to val. int snap() takes a snapshot of the array and returns the snapid: the total number of times we called snap() minus 1. int get(index, snapid) returns the value at the given index, at the time we took the snapshot with the given snapid