HackerRank - Cracking the Code Interview - Stacks: Balanced Brackets (https://www.hackerrank.com/challenges/ctci-balanced-brackets)

Solution.java

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {
    
    private static List<Character> openChars = new ArrayList<Character>(Arrays.asList('{', '(', '['));
    private static List<Character> closeChars = new ArrayList<Character>(Arrays.asList('}', ')', ']'));
    
    public static boolean isBalanced(String expression) {
      if (expression == null) return true;
        
      Stack<Character> stack = new Stack<Character>();
         
      for (char c : expression.toCharArray()) {
        if (openChars.contains(c)) stack.push(c);
        
        if (closeChars.contains(c)) {
            if (stack.empty()) return false;
            char openChar = stack.pop();
            if (!matches(openChar, c)) return false;
        }
      }
         
      return stack.empty();
   }
                                                    
   private static boolean matches(char c1, char c2) {
     if (c1 == '(' && c2 == ')')
       return true;
     else if (c1 == '{' && c2 == '}')
       return true;
     else if (c1 == '[' && c2 == ']')
       return true;
     else
       return false;
   }
  
   public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int t = in.nextInt();
        for(int a0 = 0; a0 < t; a0++) {
            String expression = in.next();
             boolean answer = isBalanced(expression);
             if(answer)
              System.out.println("YES");
             else System.out.println("NO");
        }
    }
}