1146.Snapshot_Array.py

"""
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 <= 50000
At most 50000 calls will be made to set, snap, and get.
0 <= index < length
0 <= snap_id < (the total number of times we call snap())
0 <= val <= 10^9
"""

class SnapshotArray:

    def __init__(self, length: int):
        self.idx_to_val = defaultdict(int)
        self.snapshots = []         #stores the ind_to_val dictionary at each time
        
    def set(self, index: int, val: int) -> None:
        self.idx_to_val[index] = val

    def snap(self) -> int:
        self.snapshots.append(self.idx_to_val.copy())     #now it is O(N), which is << O(L) if sparse，这里容易忘记是深拷贝
        return len(self.snapshots) - 1

    def get(self, index: int, snap_id: int) -> int:
        return self.snapshots[snap_id][index]

# 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)