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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

实现支持下列接口的「快照数组」- SnapshotArray：

• SnapshotArray(int length) - 初始化一个与指定长度相等的 类数组 的数据结构。初始时，每个元素都等于 0。
• void set(index, val) - 会将指定索引 index 处的元素设置为 val。
• int snap() - 获取该数组的快照，并返回快照的编号 snap_id（快照号是调用 snap() 的总次数减去 1）。
• int get(index, snap_id) - 根据指定的 snap_id 选择快照，并返回该快照指定索引 index 的值。

示例：

输入：["SnapshotArray","set","snap","set","get"]
     [[3],[0,5],[],[0,6],[0,0]]
输出：[null,null,0,null,5]
解释：
SnapshotArray snapshotArr = new SnapshotArray(3); // 初始化一个长度为 3 的快照数组
snapshotArr.set(0,5);  // 令 array[0] = 5
snapshotArr.snap();  // 获取快照，返回 snap_id = 0
snapshotArr.set(0,6);
snapshotArr.get(0,0);  // 获取 snap_id = 0 的快照中 array[0] 的值，返回 5

提示：

• 1 <= length <= 50000
• 题目最多进行50000 次set，snap，和 get的调用 。
• 0 <= index < length
• 0 <= snap_id < 我们调用 snap() 的总次数
• 0 <= val <= 10^9

 1 class SnapshotArray {
 2 
 3     private var dict = [Int: Int]()
 4     private var snapshots = [[Int: Int]]()
 5     
 6     init(_ length: Int) {
 7         //array = Array(repeating: 0, count: length + 1)
 8     }
 9     
10     func set(_ index: Int, _ val: Int) {
11         dict[index] = val
12     }
13     
14     func snap() -> Int {
15         snapshots.append(dict)
16         return snapshots.count - 1
17     }
18     
19     func get(_ index: Int, _ snap_id: Int) -> Int {
20         return snapshots[snap_id][index] ?? 0
21     }
22 }
23 
24 /**
25  * Your SnapshotArray object will be instantiated and called as such:
26  * let obj = SnapshotArray(length)
27  * obj.set(index, val)
28  * let ret_2: Int = obj.snap()
29  * let ret_3: Int = obj.get(index, snap_id)
30  */

 1 class SnapshotArray {
 2     var vv:[[Int:Int]] = [[Int:Int]]()
 3 
 4     init(_ length: Int) {
 5         vv.append([Int:Int]())        
 6     }
 7     
 8     func set(_ index: Int, _ val: Int) {
 9         vv[vv.count - 1][index] = val
10     }
11     
12     func snap() -> Int {
13         vv.append([Int:Int]()) 
14         return vv.count - 2
15     }
16     
17     func get(_ index: Int, _ snap_id: Int) -> Int {
18         for i in stride(from:snap_id,through:0,by:-1)
19         {
20             if vv[i][index] != nil
21             {
22                 return vv[i][index]!
23             }
24         }
25         return 0
26     }
27 }
28 
29 /**
30  * Your SnapshotArray object will be instantiated and called as such:
31  * let obj = SnapshotArray(length)
32  * obj.set(index, val)
33  * let ret_2: Int = obj.snap()
34  * let ret_3: Int = obj.get(index, snap_id)
35  */