1
2
3
4
5
6
7
8
9
10
a
b
c
d
1
2
3
4
5
6
7
8
9
10
{"", "a", "b", "c", "aa", "ab", "ac", "ba", "bb", "bc"}
{1, 2, 3, 4, 5, ...}
---
1
2
3
---
a
b
c
1
d