rs: add «1291. Sequential Digits»

Signed-off-by: Matej Focko <me@mfocko.xyz>

rs/sequential-digits.rs

@@ -0,0 +1,23 @@
+impl Solution {
+    pub fn sequential_digits(low: i32, high: i32) -> Vec<i32> {
+        let mut nums = vec![];
+
+        for d in 1..=9 {
+            let mut num = d;
+
+            let mut next_d = d + 1;
+            while num <= high && next_d <= 9 {
+                num = 10 * num + next_d;
+
+                if low <= num && num <= high {
+                    nums.push(num);
+                }
+
+                next_d += 1;
+            }
+        }
+
+        nums.sort();
+        nums
+    }
+}