1146. Snapshot Array 

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class SnapshotArray {

    private final TreeMap<Integer, Integer>[] treeMaps;
    private int snapId;

    public SnapshotArray(int length) {
        this.treeMaps = new TreeMap[length];
        for (int i = 0; i < length; i++) {
            this.treeMaps[i] = new TreeMap<>();
            this.treeMaps[i].put(0, 0);
        }
        this.snapId = 0;
    }

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

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

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

}

Binary Search

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class SnapshotArray {

    private int snapId;
    private final Map<Integer, List<int[]>> map;

    public SnapshotArray(int length) {
        this.map = new HashMap<>();
    }

    public void set(int index, int val) {
        List<int[]> pairList = map.get(index);
        if (pairList == null) {
            pairList = new ArrayList<>();
            pairList.add(new int[]{val, snapId});
            map.put(index, pairList);
        } else {
            int[] lastPair = pairList.get(pairList.size() - 1);
            if (lastPair[1] == snapId) {
                lastPair[0] = val;
            } else {
                pairList.add(new int[]{val, snapId});
            }
        }
    }

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

    public int get(int index, int snap_id) {
        List<int[]> pairList = map.get(index);
        if (pairList == null) {
            return 0;
        }

        int left = 0, right = pairList.size() - 1;
        while (left <= right) {
            int mid = (left + right) >>> 1;
            if (pairList.get(mid)[1] < snap_id) {
                left = mid + 1;
            } else if (pairList.get(mid)[1] > snap_id) {
                right = mid - 1;
            } else {
                return pairList.get(mid)[0];
            }
        }

        // now: right, left
        return right == -1 ? 0 : pairList.get(right)[0];
    }

}

References

1146. Snapshot Array