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1146. Snapshot Array

Description

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 * 104
  • 0 <= index < length
  • 0 <= val <= 109
  • 0 <= snap_id < (the total number of times we call snap())
  • At most 5 * 104 calls will be made to set, snap, and get.

Solutions

We maintain an array of length length. Each element in the array is a list, which is used to store the value set each time and the corresponding snapshot ID.

When the set method is called, we add the value and snapshot ID to the list at the corresponding index. The time complexity is $O(1)$.

When the snap method is called, we first increment the snapshot ID, then return the snapshot ID minus one. The time complexity is $O(1)$.

When the get method is called, we use binary search to find the first snapshot ID greater than snap_id at the corresponding position, and then return the previous value. If it cannot be found, return 0. The time complexity is $O(\log n)$.

The space complexity is $O(n)$.

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class SnapshotArray:

    def __init__(self, length: int):
        self.arr = [[] for _ in range(length)]
        self.i = 0

    def set(self, index: int, val: int) -> None:
        self.arr[index].append((self.i, val))

    def snap(self) -> int:
        self.i += 1
        return self.i - 1

    def get(self, index: int, snap_id: int) -> int:
        i = bisect_left(self.arr[index], (snap_id, inf)) - 1
        return 0 if i < 0 else self.arr[index][i][1]


# Your SnapshotArray object will be instantiated and called as such:
# obj = SnapshotArray(length)
# obj.set(index,val)
# param_2 = obj.snap()
# param_3 = obj.get(index,snap_id)

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class SnapshotArray {
    private List<int[]>[] arr;
    private int idx;

    public SnapshotArray(int length) {
        arr = new List[length];
        Arrays.setAll(arr, k -> new ArrayList<>());
    }

    public void set(int index, int val) {
        arr[index].add(new int[] {idx, val});
    }

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

    public int get(int index, int snap_id) {
        int l = 0, r = arr[index].size();
        while (l < r) {
            int mid = (l + r) >> 1;
            if (arr[index].get(mid)[0] > snap_id) {
                r = mid;
            } else {
                l = mid + 1;
            }
        }
        --l;
        return l < 0 ? 0 : arr[index].get(l)[1];
    }
}

/**
 * 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);
 */

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class SnapshotArray {
public:
    SnapshotArray(int length) {
        arr.resize(length);
    }

    void set(int index, int val) {
        arr[index].emplace_back(i, val);
    }

    int snap() {
        return i++;
    }

    int get(int index, int snap_id) {
        auto it = upper_bound(arr[index].begin(), arr[index].end(), make_pair(snap_id, INT_MAX));
        return it == arr[index].begin() ? 0 : prev(it)->second;
    }

private:
    vector<vector<pair<int, int>>> arr;
    int i = 0;
};

/**
 * Your SnapshotArray object will be instantiated and called as such:
 * SnapshotArray* obj = new SnapshotArray(length);</