1146. 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 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 {
public:
    SnapshotArray(int length) : snap_num(0), m_container(length) { // time: O(length)
        
    }
    
    void set(int index, int val) { // time: O(log(snap_num))
        m_container[index][snap_num] = val;
    }
    
    int snap() {
        return snap_num++;
    }
    
    int get(int index, int snap_id) { // time: O(log(snap_num))
        auto it = m_container[index].upper_bound(snap_id);
        return it == m_container[index].begin() ? 0 : prev(it)->second;
    }
private:
    int snap_num;
    vector<map<int, int> > m_container; // only records changes by set()
};