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pub fn combinations(
n: usize,
k: usize,
) -> Vec<Vec<usize>> {
assert!(k <= n);
let mut res = vec![];
let mut a: Vec<_> = (0..k).collect();
loop {
res.push(a.clone());
let mut i = k;
for j in (0..k).rev() {
if a[j] != j + n - k {
i = j;
break;
}
}
if i == k {
return res;
}
a[i] += 1;
for j in i + 1..k {
a[j] = a[j - 1] + 1;
}
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test() {
let res = combinations(5, 3);
assert_eq!(
res,
[
[0, 1, 2],
[0, 1, 3],
[0, 1, 4],
[0, 2, 3],
[0, 2, 4],
[0, 3, 4],
[1, 2, 3],
[1, 2, 4],
[1, 3, 4],
[2, 3, 4]
]
);
}
}