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# 1146.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"]
[,[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 = 5
snapshotArr.snap(); // Take a snapshot, return snap_id = 0
snapshotArr.set(0,6);
snapshotArr.get(0,0); // Get the value of array 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)
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);
*/