java版本

class SnapshotArray {
    int id = 0;
    List<int[]>[] snapshots;
    public SnapshotArray(int length) {
        snapshots = new List[length];
        for (int i = 0; i < length; i++) {
            snapshots[i] = new ArrayList<int[]>();
        }
    }

    public void set(int index, int val) {
        snapshots[index].add(new int[]{id, val});
    }

    public int snap() {
        int curr = id;
        id++;
        return curr;
    }



    public int get(int index, int snap_id) {
        List<int[]> snaplist = snapshots[index];
        int low = -1, high = snaplist.size() - 1;
        while (low < high) {
            int mid = low + (high - low + 1) / 2;
            if (snaplist.get(mid)[0] <= snap_id) {
                low = mid;
            } else {
                high = mid - 1;
            }
        }
        return low >= 0 ? snaplist.get(low)[1] : 0;
    }

}

c++版本

class SnapshotArray {
public:
    int snap_id=0;
    unordered_map<int,vector<pair<int,int>>> history;
    SnapshotArray(int length) {

    }
    
    void set(int index, int val) {
        history[index].emplace_back(snap_id,val);
    }
    
    int snap() {
        return snap_id++;
    }
    
    int get(int index, int snap_id) {
        auto &h = history[index];
        int l = 0, r = h.size()-1, mid,target = snap_id+1;
        while(l <= r){
            mid = l +((r-l)>>1);
            if(h[mid].first < target){
                l = mid+1;
            }else{
                r = mid-1;
            }
        }
        return l-1 >= 0 ? h[l-1].second:0;
    }
};