1146. Snapshot Array

Problem

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 <= 5 * 10^4

• 0 <= index < length

• 0 <= val <= 10^9

• 0 <= snap_id < (the total number of times we call snap())

• At most 5 * 10^4 calls will be made to set, snap, and get.

Solution (Java)

class SnapshotArray {
    private int snapId = -1;
    private final Map<Integer, TreeMap<Integer, Integer>> indexToSnapMap;
    private final int[] ar;

    public SnapshotArray(int length) {
        indexToSnapMap = new HashMap<>();
        ar = new int[length];
    }

    public void set(int index, int val) {
        if (indexToSnapMap.containsKey(index)) {
            if (!indexToSnapMap.get(index).containsKey(snapId)) {
                indexToSnapMap.get(index).put(snapId, ar[index]);
            }
        } else {
            TreeMap<Integer, Integer> snapToValueMap = new TreeMap<>();
            snapToValueMap.put(snapId, ar[index]);
            indexToSnapMap.put(index, snapToValueMap);
        }
        ar[index] = val;
    }

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

    public int get(int index, int snapId) {
        if (indexToSnapMap.containsKey(index)) {
            Map.Entry<Integer, Integer> value = indexToSnapMap.get(index).ceilingEntry(snapId);
            if (value != null) {
                return value.getValue();
            }
        }
        return ar[index];
    }
}

/**
 * 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);
 */

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).