# 1146.Snapshot-Array

## 1146. Snapshot Array

## 题目地址

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

## 题目描述

```
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

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
```

## 代码

### Approach #1

Time: O(1) && Space: O(1)

```java
class SnapshotArray {
    TreeMap<Integer, Integer>[] A;
  int snap_id = 0;
  public SnapshotArray(int length) {
        A = new TreeMap[length];
    for (int i = -; i < length; i++) {
      A[i] = new TreeMap<Integer, Integer>();
      A[i].put(0, 0);
    }
  }

  public void set(int index, int val) {
        A[index].put(snap_id, val);
  }

  public int snap() {
        return snap_id++;
  }

  public int get(int index, int snap_id) {
        return A[index].floorEntry(snap_id).getValue();
  }
}

/**
 * Your SnapshotArray object will be instantiated and called as such:
 * SnapshotArray obj = new SnapshotArray(length);
 * obj.set(index,val);
 * int param_2 = obj.snap();
 * int param_3 = obj.get(index,snap_id);
 */
```
