Solve valid-parenthesis.

source/lib.rs

@@ -2,3 +2,4 @@ pub mod longest_common_prefix;
 pub mod palindrome_number;
 pub mod reverse_integer;
 pub mod two_sum;
+pub mod valid_parenthesis;

source/valid_parenthesis/mod.rs

@@ -0,0 +1,55 @@
+pub fn is_valid(string: String) -> bool {
+  // If the input string is of an odd length it cannot be valid, since every
+  // opening character must also have a closing one.
+  if string.len() % 2 != 0 {
+    return false;
+  }
+
+  fn get_closing_character(character: char) -> char {
+    match character {
+      '[' => ']',
+      '{' => '}',
+      '(' => ')',
+      _ => panic!("Invalid input character: {character}"),
+    }
+  }
+
+  let mut stack = vec![];
+
+  for character in string.chars() {
+    match character {
+      // For any opening characters, add their counterpart to the stack and move
+      // on to the next character in the string.
+      '[' | '{' | '(' => stack.push(get_closing_character(character)),
+
+      // For any other characters, get the most recent character we've seen and
+      // if it doesn't equal the expected closing character then the string is
+      // invalid.
+      _ => {
+        if character != stack.pop().unwrap_or_default() {
+          return false;
+        }
+      }
+    };
+  }
+
+  // After the string has been looped over, if anything is left in the stack the
+  // string is invalid.
+  stack.len() == 0
+}
+
+#[test]
+fn test_valid_parenthesis() {
+  assert!(is_valid("".to_string()));
+  assert!(is_valid("[]".to_string()));
+  assert!(is_valid("[()]".to_string()));
+  assert!(is_valid("[{}()]".to_string()));
+
+  assert!(!is_valid("[".to_string()));
+  assert!(!is_valid("[()".to_string()));
+  assert!(!is_valid("((".to_string()));
+
+  assert!(!is_valid("[)".to_string()));
+  assert!(!is_valid("[(}]".to_string()));
+  assert!(!is_valid("([{}()]}".to_string()));
+}