different ways to add parentheses

HashMap<String, List<Integer>> map = new HashMap<>();
public List<Integer> (String input) {
    if (map.containsKey(input))
        return map.get(input);
    List<Integer> ans = new ArrayList<>();
    if (input == null || input.length() == 0)
        return ans;
    for (int i = 0; i < input.length(); ++i) {
        char c = input.charAt(i);
        if (c == '+' || c == '-' || c == '*') {
            List<Integer> left = diffWaysToCompute(input.substring(0, i));
            List<Integer> right = diffWaysToCompute(input.substring(i + 1));
            for (int x : left) {
                for (int y : right) {
                    int tmp = 0;
                    switch(c) {
                        case '+':
                            tmp = x + y;
                            break;
                        case '-':
                            tmp = x - y;
                            break;
                        case '*':
                            tmp = x * y;
                            break;
                        default:
                            break;
                    }
                    ans.add(tmp);
                }
            }
        }
    }
    if (ans.size() == 0)
        ans.add(Integer.parseInt(input));
    map.put(input, ans);
    return ans;
}