1146. Snapshot Array

https://leetcode.com/problems/snapshot-array

Description

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

ac