LeetCode 1146: Snapshot Array

Question

I'm posting my C++ code for LeetCode's Snapshot Array. If you have time and would like to review, please do so. Thank you!

Problem

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

LeetCode Template

class SnapshotArray {
public:
    SnapshotArray(int length) {
        
    }
    
    void set(int index, int val) {
        
    }
    
    int snap() {
        
    }
    
    int get(int index, int snap_id) {
        
    }
};

/**
 * 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);
 */

Accepted C++

class SnapshotArray {
public:
    SnapshotArray(const int array_length) {}
    int curr_shot_id = 0;
    std::unordered_map<int, std::vector<pair<int, int>>> id_map;

    // Returns current snapshot O(1)
    const int snap() {
        return curr_shot_id++;
    }

    // Setter with unordered_map
    const void set(const int key, const int shot_id) {
        if (id_map[key].empty() || id_map[key].back().first != curr_shot_id) {
            id_map[key].push_back({curr_shot_id, shot_id});

        } else {
            id_map[key].back().second = shot_id;
        }
    }

    // Getter with binary searching -> O(1) memory O(log N) time
    const int get(const int key, const int shot_id) {
        const auto iter = std::upper_bound(id_map[key].begin(), id_map[key].end(), std::pair<int, int>(shot_id, INT_MAX));
        return iter == std::begin(id_map[key]) ? 0 : std::prev(iter)->second;
    }
};

Reference

LeetCode is a platform only for interviewing and competitive programming. On LeetCode, there is a class usually named Solution (for this post is SnapshotArray) with one or more public functions which we are not allowed to rename.

• Question
• Discussion

Answer 1

The class template provided provides the necessary public interface, anything else should be private rather than public. Therefore the variable int curr_shot_id and the variable std::unordered_map<int, std::vector<pair<int, int>>> id_map; should be declared after private:.

The variable curr_shot_id should be initialized by the constructor SnapshotArray(int length).

    SnapshotArray(int length)
    : curr_shot_id{0}
    { }

It's not clear that you need a binary search of the map since a map is a direct access memory structure which means that the key will be hashed and that provides a direct reference to the object stored.