[LC] 1146. Snapshot Array

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

class SnapshotArray {
    
    List<TreeMap<Integer, Integer>> list;
    int shots;
    public SnapshotArray(int length) {
        list = new ArrayList<>();
        for (int i = 0; i < length; i++) {
            list.add(new TreeMap<>());
            list.get(i).put(0, 0);
        }
    }
    
    public void set(int index, int val) {
        list.get(index).put(shots, val);
    }
    
    public int snap() {
        return shots++;
    }
    
    public int get(int index, int snap_id) {
        return list.get(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);
 */