Solve roman-to-integer.

source/lib.rs

@@ -2,5 +2,6 @@ pub mod implement_strstr;
 pub mod longest_common_prefix;
 pub mod palindrome_number;
 pub mod reverse_integer;
+pub mod roman_to_integer;
 pub mod two_sum;
 pub mod valid_parenthesis;

source/roman_to_integer/mod.rs

@@ -0,0 +1,44 @@
+pub fn roman_to_int(string: String) -> i32 {
+  let mut characters = string.chars().peekable();
+  let mut number = 0;
+
+  while let Some(character) = characters.next() {
+    let value = match character {
+      'I' => 1,
+      'V' => 5,
+      'X' => 10,
+      'L' => 50,
+      'C' => 100,
+      'D' => 500,
+      'M' => 1000,
+      _ => panic!("Unknown character: {character}"),
+    };
+
+    if let Some(next) = characters.peek() {
+      let subtract = match next {
+        'V' | 'X' if character == 'I' => true,
+        'L' | 'C' if character == 'X' => true,
+        'D' | 'M' if character == 'C' => true,
+        _ => false,
+      };
+
+      if subtract {
+        number -= value;
+        continue;
+      }
+    }
+
+    number += value;
+  }
+
+  number
+}
+
+#[test]
+fn test_roman_to_integer() {
+  assert_eq!(roman_to_int("III".to_string()), 3);
+  assert_eq!(roman_to_int("LVIII".to_string()), 58);
+  assert_eq!(roman_to_int("MCMXCIV".to_string()), 1994);
+  assert_eq!(roman_to_int("XX".to_string()), 20);
+  assert_eq!(roman_to_int("DCXXI".to_string()), 621);
+}