Only 3 pairs, move away from HashSet

src/lib.rs

@@ -1,21 +1,13 @@
 pub fn brackets_are_balanced(string: &str) -> bool {
-    use std::collections::HashMap;
-
-    let pairs: HashMap<char, char> = [(')', '('), ('}', '{'), (']', '[')]
-        .iter().cloned().collect();
         
     let mut stack = vec![];
 
     for char in string.chars() {
         match char {
             '(' | '{' | '[' => stack.push(char),
-            ')' | '}' | ']' => {
-                if let Some(left) = stack.pop() {
-                    if let Some(right) = pairs.get(&char) {
-                        if left != *right { return false }
-                    }
-                } else { return false }
-            },
+            ')' => if stack.pop() != Some('(') { return false },
+            '}' => if stack.pop() != Some('{') { return false },
+            ']' => if stack.pop() != Some('[') { return false },
              _  => {},
         }
     }