Add Rust permutations (#196)

rust/Cargo.toml

@@ -4,6 +4,10 @@ version = "0.1.0"
 
 edition = "2021"
 
+[[bin]]
+name = "permutations"
+path = "combinatorics/permutations.rs"
+
 [[bin]]
 name = "topological_sort"
 path = "graphs/dfs/topological_sort.rs"

rust/combinatorics/permutations.rs

@@ -0,0 +1,20 @@
+pub fn next_permutation(p: &mut [usize]) -> Option<()> {
+    let a = (0..p.len() - 1).rfind(|&i| p[i] < p[i + 1])?;
+    let b = (a + 1..p.len()).rfind(|&i| p[a] < p[i]).unwrap();
+    p.swap(a, b);
+    p[a + 1..].reverse();
+    Some(())
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::next_permutation;
+
+    #[test]
+    fn basic_test() {
+        let mut p: [usize; 4] = [0, 3, 2, 1];
+        let res = next_permutation(&mut p);
+        assert_eq!(p, [1, 0, 2, 3]);
+        assert_eq!(res, Some(()));
+    }
+}