Question 1. Balanced Brackets

static String isBalanced(String s) {
    if(s == null){
        return "NO";
    }
    Stack<Character> stack = new Stack<Character>();

    for(char c : s.toCharArray()){
        if(c == '{' || c== '[' || c == '('){
            stack.push(c);
        }else if(!stack.isEmpty()){
            if(c == '}' && stack.peek()=='{'){
                stack.pop();
            }else if(c == ']' && stack.peek() =='['){
                stack.pop();
            }else if(c == ')' && stack.peek() =='('){
                stack.pop();
            }
        }else{
            return "NO";
        }
    }
    return stack.isEmpty() ? "YES" : "NO";        
}

Question 2. Print tree left to