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:

1SnapshotArray snapshotArr = new SnapshotArray(3); // set the length to be 3
2
3snapshotArr.set(0,5);  // Set array[0] = 5
4
5snapshotArr.snap();  // Take a snapshot, return snap_id = 0
6
7snapshotArr.set(0,6);
8
9snapshotArr.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.

Solution

We wish to find the pos for the most recent value at the time we took the snapshot with the given snap_id, we are trying to find the rightmost index in history=histories[i] such that the snap_id at history[pos] is less or equal to the target snap_id (a[i][0] <= snap_id). This means that the feasible function is a[i][0] <= snap_id, whenever this is true, we must check the positions on its right to find the rightmost position that makes this condition hold.

Implementation

1class SnapshotArray(object):
2
3def __init__(self, n):
4    # set up histories so that each index has its own history
5    self.histories = [[[-1, 0]] for _ in range(n)]
6    self.snap_id = 0
7
8def set(self, index, val):
9    self.histories[index].append([self.snap_id, val])
10
11def snap(self):
12    self.snap_id += 1
13    return self.snap_id - 1
14
15def get(self, index, snap_id):
16    left, right, pos = 0, len(self.histories[index])-1, -1
17    while left <= right:
18        mid = (left+right) // 2
19        if self.histories[index][mid][0] <= snap_id:
20            left = mid + 1
21            pos = mid
22        else:
23            right = mid - 1
24    return self.histories[index][pos][1]

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