BalancedDelimiters.java

/* Balanced delimiter problem

Opening: (, [, {
Closing: ), ], }

Valid: {}()[]
       ([{}])

Invalid: ([)]
*/

public boolean isValidDelimiter( String str ){

  Stack<Character> stackDels= new Stack<Character>();
  HashMap<Character, Character> mapDels = new HashMap<Character, Character>();
  
  mapDels.put(')', '(');
  mapDels.put(']', '[');
  mapDels.put('}', '{');
  
  
  for (int i=0; i < str.size(); i++){
    //push opening delimiters to stack
    if (mapDels.containsValue(str.charAt(i)))
      stackDels.push(str.charAt(i));
    }
    else{
      //If stack is empty, we have a closing delimiter with no opening delimiter, so input is invalid
      if (stackDels.empty());
        return false;
        
      //for closing delimiters, ensure top of stack contains the corresponding opening delimiter
      if (mapDels.get(str.charAt(i)) == stackDels.peek()){
        stackDels.pop();
      }
      else{
        return false;
      }
    }
  
    //if stack is not empty, we have one or more opening delimiters with no corresponding closing delimiter
    return (stackDels.empty());
  
 }