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

Solution 1: Array + Binary Search

Maintain an array, each element of which is a list storing the values set each time and their corresponding snapshot IDs.

Each time a value is set, add the value and snapshot ID to the list at the corresponding index.

Each time a value is retrieved, use binary search to find the first value in the corresponding position that is greater than the snapshot ID snap_id, and then return the previous value.

In terms of time complexity, the time complexity of setting a value is $O(1)$, the time complexity of a snapshot is $O(1)$, and the time complexity of getting a value is $O(\log n)$.

Python Code
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class SnapshotArray:
    def __init__(self, length: int):
        self.idx = 0
        self.arr = defaultdict(list)

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

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

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

Java Code
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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) {
        var vals = arr[index];
        int left = 0, right = vals.size();
        while (left < right) {
            int mid = (left + right) >> 1;
            if (vals.get(mid)[0] > snap_id) {
                right = mid;
            } else {
                left = mid + 1;
            }
        }
        return left == 0 ? 0 : vals.get(left - 1)[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);
 */

C++ Code
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class SnapshotArray {
public:
    SnapshotArray(int length) {
        idx = 0;
        arr = vector<vector<pair<int, int>>>(length);
    }

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

    int snap() {
        return idx++;
    }

    int get(int index, int snap_id) {
        auto& vals = arr[index];
        int left = 0, right = vals.size();
        while (left < right) {
            int mid = (left + right) >> 1;
            if (vals[mid].first > snap_id) {
                right = mid;
            } else {
                left = mid + 1;
            }
        }
        return left == 0 ? 0 : vals[left - 1].second;
    }

private:
    vector<vector<pair<int, int>>> arr;
    int idx;
};

/**
 * 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);
 */

Go Code
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type SnapshotArray struct {
	idx int
	arr [][]pair
}

func Constructor(length int) SnapshotArray {
	return SnapshotArray{0, make([][]pair, length)}
}

func (this *SnapshotArray) Set(index int, val int) {
	this.arr[index] = append(this.arr[index], pair{this.idx, val})
}

func (this *SnapshotArray) Snap() int {
	this.idx++
	return this.idx - 1
}

func (this *SnapshotArray) Get(index int, snap_id int) int {
	vals := this.arr[index]
	i := sort.Search(len(vals), func(i int) bool { return vals[i].i > snap_id })
	if i == 0 {
		return 0
	}
	return vals[i-1].v
}

type pair struct{ i, v int }

/**
 * Your SnapshotArray object will be instantiated and called as such:
 * obj := Constructor(length);
 * obj.Set(index,val);
 * param_2 := obj.Snap();
 * param_3 := obj.Get(index,snap_id);
 */