Snapshot Array

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 <= 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.

Code

Time Complexity: $O(n\log n)$, Space Complexity: $O(n)$

建立 base case：

for(int i = 0; i < length; i++) {
	map<int, int> mp;
	mp[0] = 0;
	A[i] = mp;
}

如此一來在 call

return prev(A[index].upper_bound(snap_id))->second;

時就不用擔心 prev 之後會沒有值可以取（map 有可能是空的）。

class SnapshotArray {
public:
    vector<map<int, int>> A;
    int snap_id;
    SnapshotArray(int length) {
        A.resize(length);
        for(int i = 0; i < length; i++) {
            map<int, int> mp;
            mp[0] = 0;
            A[i] = mp;
        }
        
        snap_id = 0;
    }
    
    void set(int index, int val) {
        A[index][snap_id] = val;
    }
    
    int snap() {
        return snap_id++;
    }
    
    int get(int index, int snap_id) {
        return prev(A[index].upper_bound(snap_id))->second;
    }
};
 
/**
 * Your SnapshotArray object will be instantiated and called as such:
 * SnapshotArray* obj = new SnapshotArray(length);
 * obj->set(index,val);
 * int param_2 = obj->snap();
 * int param_3 = obj->get(index,snap_id);
 */

Source

• Snapshot Array - LeetCode