diff options
author | jack <[email protected]> | 2024-06-14 13:13:15 -0600 |
---|---|---|
committer | jack <[email protected]> | 2024-06-14 13:13:15 -0600 |
commit | d343a3b18dbaca9297d4f7b2280c4a16f0a5c8aa (patch) | |
tree | 96df922dae2a5073d9a50ea70c7aae6aa37e3ebe /count_good_numbers.c | |
download | leetcode-d343a3b18dbaca9297d4f7b2280c4a16f0a5c8aa.tar.gz leetcode-d343a3b18dbaca9297d4f7b2280c4a16f0a5c8aa.zip |
initial
Diffstat (limited to 'count_good_numbers.c')
-rw-r--r-- | count_good_numbers.c | 32 |
1 files changed, 32 insertions, 0 deletions
diff --git a/count_good_numbers.c b/count_good_numbers.c new file mode 100644 index 0000000..86432c4 --- /dev/null +++ b/count_good_numbers.c @@ -0,0 +1,32 @@ +// 1922. Count Good Numbers +#include "leetcode.h" + +/* + * a digit string is good if the digits (0-indexed) at even indices are even and + * the digits at odd indices are prime (2,3,5,or 7). for example 2582 is good + * because the digits 2 and 8 at even positions are even and the digits 5 and 2 + * at odd positions are prime. however, 3245 is not good because 3 is at an even + * index, but is not even. given an integer 'n', return the total number of good + * digit strings of length 'n'. since the answer may be large, return it modulo + * 1e9+7. a digit string is a string consisting of digits 0 through 9 that may + * contain leading zeros. + */ + +const int MOD = 1e9 + 7; + +long long _pow(long long a, long long b) { + if (!b) + return 1; + return b % 2 ? a * _pow(a, b - 1) % MOD : _pow((a * a) % MOD, b / 2); +} + +int countGoodNumbers(long long n) { + long long odd = n / 2, even = n - odd; + return _pow(20, odd) * ((n % 2) ? 5 : 1) % MOD; +} + +int main() { + printf("%d\n", countGoodNumbers(1)); // expect: 5 + printf("%d\n", countGoodNumbers(4)); // expect: 400 + printf("%d\n", countGoodNumbers(50)); // expect: 564908303 +} |