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 <= 50000`
- At most `50000` calls will be made to `set`, `snap`, and `get`.
- `0 <= index < length`
- `0 <= snap_id <` (the total number of times we call `snap()`)
- `0 <= val <= 10^9`