InfixToPostFix conversion

infixToPostFix.java

private static String infixToPostfix(final String equation){
    Stack<Character> stack = new Stack<Character>();
    StringBuilder postFixNotation = new StringBuilder();
    HashMap<Character, Integer> operators = new HashMap<>();
    operators.put('^', 5);
    operators.put('/', 4);
    operators.put('*', 3);
    operators.put('+', 2);
    operators.put('-', 1);
    operators.put('(', 0);

    Set<Character> operator = operators.keySet();


    int pointer = 0;
    char[] equationChars = equation.toCharArray();
    while (pointer < equationChars.length) {
        if (equationChars[pointer] == ')'){
            for (int i=0; i < stack.size() - 1; i++) {
                postFixNotation.append(stack.pop());
            }
            pointer++;
            boolean checkNextCharContainsOperator = operators.containsKey(equationChars[pointer]);
            if (checkNextCharContainsOperator &&
                    operators.get(stack.peek()) > operators.get(equationChars[pointer])) {
                postFixNotation.append(stack.pop());
                stack.push(equationChars[pointer]);
            }
            pointer++;
            continue;
        }
        if (operator.contains(equationChars[pointer])) {
            stack.push(equationChars[pointer]);
        } else if (equationChars[pointer] != '('){
            postFixNotation.append(equationChars[pointer]);
        }
        pointer++;
    }

    for (int i=0; i< stack.size(); i++) {
        postFixNotation.append(stack.pop());
    }

    return postFixNotation.toString();
}