GOOGLE interview question #2
Description
Design a SnapshotArray that provides the following functionality:
SnapshotArray(int length): Initializes an array-like structure of the given length, with each element initially set to 0.
void set(int index, int val): Updates the element at the specified index to the given value.
int snap(): Captures a snapshot of the current state of the array and returns the snap_id, which is the number of times snap() has been called, minus 1.
int get(int index, int snap_id): Returns the value at the specified index at the time the snapshot with the given snap_id was taken.