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. 1 <= length <= 50000.
  2. At most 50000 calls will be made to set, snap, and get.
  3. 0 <= index < length
  4. 0 <= snap_id < (the total number of times we call snap())
  5. 0 <= val <= 10^9

分析

题目的意思是：实现一个镜像数组，我用list和字典暴力了一下，发现超时了。看了别人的实现后，发现在一开始可以不用申请数组，直接用字典搞定，然后在镜像的时候用数组存放镜像就行了，看来我正在处于并将长期处于被虐的阶段。

代码

class SnapshotArray:

    def __init__(self, length: int):
        self.arr={}
        self.snap_id=0
        self.snap_arr=[]
        

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

    def snap(self) -> int:
        self.snap_arr.append(dict(self.arr))
        self.snap_id+=1
        return self.snap_id-1
        

    def get(self, index: int, snap_id: int) -> int:
        arr=self.snap_arr[snap_id]
        if(index in arr):
            return arr[index]
        else:
            return 0
        


# 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)

参考文献

​​[LeetCode] Python 3 | Three Methods | Explanations​​