Create let mut

let mut

@@ -0,0 +1,50 @@
+fn gray(n: u8) -> Vec<String> {
+    if n == 0 {
+        return vec![String::from("")];
+    }
+
+    // Start with 1-bit gray code
+    let mut result = vec![String::from("0"), String::from("1")];
+
+    // Build for n bits
+    for i in 2..=n {
+        let size = result.len();
+        // Reflect the current list
+        for j in (0..size).rev() {
+            result.push(result[j].clone());
+        }
+        // Append '0' to the first half
+        for j in 0..size {
+            result[j] = format!("0{}", result[j]);
+        }
+        // Append '1' to the second half
+        for j in size..2*size {
+            result[j] = format!("1{}", result[j]);
+        }
+    }
+
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let test_data = [
+            (0, vec![""]),
+            (1, vec!["0", "1"]),
+            (2, vec!["00", "01", "11", "10"]),
+            (3, vec!["000", "001", "011", "010", "110", "111", "101", "100"]),
+            (4, vec!["0000", "0001", "0011", "0010", "0110", "0111", "0101", "0100",
+                     "1100", "1101", "1111", "1110", "1010", "1011", "1001", "1000"]),
+        ];
+
+        test_data
+            .iter()
+            .for_each(|(n, out)| 
+                assert_eq!(gray(*n), *out)
+            );
+    }
+}