using real Iterator<Integer> instead of fake CharacterIterator

src/main/java/Solution.java

@@ -1,42 +1,35 @@
 import java.util.*;
+import java.util.function.BiPredicate;
 
-/**
- * Template code to help you parse the standard input 
- * according to the problem statement.
- **/
 class Solution {
-	
-	public static void main( String[] args ) {
-		Scanner in = new Scanner( System.in );
-		// read values with in.next...() methods
-		String input = in.nextLine();
-		// code your solution here
-		boolean output = isInputWellFormed(input);
-		// Write result with System.out.println()
-		System.out.println( output );
-	}
-
-	private static boolean isInputWellFormed(String input) {
-		HashMap<Character, Character> validPairs = new HashMap<>();
-		validPairs.put('{', '}');
-		validPairs.put('(', ')');
-		validPairs.put('[', ']');
-
-		Deque<Character> unmatchedChars = new ArrayDeque<>();
-
-		for (int i = 0; i < input.length(); i++) {
-			char currentChar = input.charAt(i);
-
-			if (validPairs.keySet().contains(currentChar)) {
-				unmatchedChars.push(currentChar); // Add only opening ({[ to the unmatchedChars.
-			} else if (validPairs.values().contains(currentChar)) {
-				if (!unmatchedChars.isEmpty() && validPairs.get(unmatchedChars.peekFirst()) == currentChar) {
-					unmatchedChars.pop(); // Match found, empty the unmatchedChars.
-				} else {
-					return false;
-				}
-			}
-		}
-		return unmatchedChars.isEmpty();
-	}
+    
+    // best readable version, also without dark ASCII code tricks
+    private static final BiPredicate<Integer, Integer> BRACKETS_MATCH = ( c1, c2 ) -> ( c1 != null && c2 != null )
+            && (( c1 == '(' && c2 == ')' ) || ( c1 == '[' && c2 == ']' ) || ( c1 == '{' && c2 == '}' ));
+    
+    // code your solution here
+    private static boolean checkNextChar( Deque<Integer> stack, Iterator<Integer> iter ) {
+        if ( ! iter.hasNext() ) return stack.isEmpty();
+        
+        int c = iter.next();
+        if ( BRACKETS_MATCH.test( stack.peek(), c )) {
+            stack.pop();
+        }
+        else {
+            stack.push( c );
+        }
+        
+        return checkNextChar( stack, iter );
+    }
+    
+    public static void main( String[] args ) {
+        // hint: read values via Scanner methods
+        var inputLine = new Scanner( System.in ).nextLine();
+        
+        var stack = new ArrayDeque<Integer>( inputLine.length() );
+        var iter  = inputLine.chars().iterator();
+        
+        // Write result with System.out.println()
+        System.out.println( checkNextChar( stack, iter ) );
+    }
 }