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class SnapshotArray {
public:
    unordered_map<int, map<int, int>> a;
    int cur_snap = 0;

    SnapshotArray(int length) {

    }

    int snap() {
        return cur_snap++;
    }

    void set(int index, int val) {
      a[index][cur_snap] = val;
    }

    int get(int index, int snap_id) {
      auto it = a[index].upper_bound(snap_id);
      return it == begin(a[index]) ? 0 : prev(it)->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);
 */