【leetcode】1146. Snapshot Array

题目如下：

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

解题思路：用snap_id记录当前snap的版本号，每次set操作的时候，把(snap_id,index)当做key存入字典中；get操作的时候，把当前(snap_id,index)当做key在字典中查找，如果key存在则返回对应的值，如果不存在就把snap_id减去1继续查找，直到找到值或者snap_id为0为止。

代码如下：

class SnapshotArray(object):

    def __init__(self, length):
        """
        :type length: int
        """
        self.length = length
        self.dic = {}
        self.snap_id = 0

    def set(self, index, val):
        """
        :type index: int
        :type val: int
        :rtype: None
        """
        if index >=0 and index < self.length:
            self.dic[(index,self.snap_id)] = val


    def snap(self):
        """
        :rtype: int
        """
        self.snap_id += 1
        return self.snap_id - 1

    def get(self, index, snap_id):
        """
        :type index: int
        :type snap_id: int
        :rtype: int
        """
        while snap_id >= 0:
            if (index,snap_id) in self.dic:
                return self.dic[(index,snap_id)]
            else:
                snap_id -= 1
        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)