4
3
2
1
0
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Instructor solution
4.4 Recursion - Lesson
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