Snapshot Array

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 `snap_id`: the total number of times we called `snap()` minus `1`. - `int get(index, snap_id)` returns the value at the given `index`, at the time we took the snapshot with the given `snap_id`   **Example 1:** **Input:** ["SnapshotArray","set","snap","set","get"] [[3],[0,5],[],[0,6],[0,0]] **Output:** [null,null,0,null,5] **Explanation: ** SnapshotArray snapshotArr = new SnapshotArray(3); // set the length to be 3 snapshotArr.set(0,5); // Set array[0] = 5 snapshotArr.snap(); // Take a snapshot, return snap_id = 0 snapshotArr.set(0,6); snapshotArr.get(0,0); // Get the value of array[0] with snap_id = 0, return 5 ```   **Constraints:** - `1 <= length <= 5 * 104` - `0 <= index < length` - `0 <= val <= 109` - `0 <= snap_id < `(the total number of times we call `snap()`) - At most `5 * 104` calls will be made to `set`, `snap`, and `get`.