1using System;
2using System.Collections.Generic;
3
4public class SmallestInfiniteSet {
5    private SortedSet<int> set;
6    private int current;
7
8    public SmallestInfiniteSet() {
9        set = new SortedSet<int>();
10        current = 1;
11    }
12
13    public int PopSmallest() {
14        if (set.Count > 0) {
15            int smallest = set.Min;
16            set.Remove(smallest);
17            return smallest;
18        }
19        return current++;
20    }
21
22    public void AddBack(int num) {
23        if (num < current && !set.Contains(num))
24            set.Add(num);
25    }
26}
27