45 lines
999 B
Rust
45 lines
999 B
Rust
use std::io::{read_to_string, stdin};
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const MOD: usize = 1_000_000_007;
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fn get_mod_power(n: usize, p: usize) -> usize {
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let mut x = p;
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let mut curr = n % MOD;
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let mut res = 1;
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while x > 0 {
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if x & 1 == 1 {
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res *= curr;
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res %= MOD;
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}
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x >>= 1;
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curr *= curr;
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curr %= MOD;
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}
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res
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}
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fn main() {
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let temp = read_to_string(stdin()).unwrap();
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let mut iter = temp
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.split_ascii_whitespace()
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.map(|x| x.parse::<usize>().unwrap());
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let n = iter.next().unwrap();
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let k = iter.next().unwrap();
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let mut fac = vec![];
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let mut facinv = vec![];
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fac.push(1);
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facinv.push(1);
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fac.push(1);
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facinv.push(1);
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for i in 2..=n {
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let before = fac.last().unwrap();
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fac.push((before * i) % MOD);
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facinv.push(get_mod_power(*fac.last().unwrap(), MOD - 2));
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}
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let nck = (fac[n] * facinv[n - k] % MOD) * facinv[k] % MOD;
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println!("{}", nck);
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}
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