1146.Snapshot Array
===

###### tags: `Medium`,`Array`,`Binary Search`,`Hash Table`,`Design`

[1146. Snapshot Array](https://leetcode.com/problems/snapshot-array/)

### 題目描述

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`

### 範例

**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 * 10^4^
* 0 <= index < length
* 0 <= val <= 10^9^
* 0 <= snap_id < (the total number of times we call `snap()`)
* At most 5 * 10^4^ calls will be made to `set`, `snap`, and `get`.

### 解答

### Reference

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