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 givenindex
to be equal toval
.int snap()
takes a snapshot of the array and returns thesnap_id
: the total number of times we calledsnap()
minus1
.int get(index, snap_id)
returns the value at the givenindex
, at the time we took the snapshot with the givensnap_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 callsnap()
)- At most
5 * 104
calls will be made toset
,snap
, andget
.
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
class SnapshotArray(object):
def __init__(self, n):
# set up histories so that each index has its own history
self.histories = [[[-1, 0]] for _ in range(n)]
self.snap_id = 0
def set(self, index, val):
self.histories[index].append([self.snap_id, val])
def snap(self):
self.snap_id += 1
return self.snap_id - 1
def get(self, index, snap_id):
left, right, pos = 0, len(self.histories[index])-1, -1
while left <= right:
mid = (left+right) // 2
if self.histories[index][mid][0] <= snap_id:
left = mid + 1
pos = mid
else:
right = mid - 1
return self.histories[index][pos][1]
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