1146\. Snapshot Array

Medium

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`