brackets.java

import java.util.*;

public class Groups{
  private static final List<Character> OPEN = Arrays.asList('(', '{', '[');
  private static final List<Character> CLOSE = Arrays.asList(')', '}', ']');

  public static boolean groupCheck(String s){
    if (s == null || s.length() == 0) {
      return true;
    }
    Stack<Character> stack = new Stack<>();
    for (int i = 0; i < s.length(); i++) {
      char current = s.charAt(i);
      if (isOpen(current)) {
        stack.push(current);
      } else {
        if (stack.isEmpty()) {
          return false;
        }
        char prev = stack.peek();
        if (isMatch(prev, current)) {
          stack.pop();
        }
      }
    }
    return stack.isEmpty();
  }
  
  private static boolean isOpen(char c) {
    return OPEN.contains(c);
  }
  
  private static boolean isClose(char c) {
    return CLOSE.contains(c);
  }
  
  private static boolean isMatch(char prev, char next) {
    return isOpen(prev) && (OPEN.indexOf(prev) == CLOSE.indexOf(next));
  }
  
}