241. Different Ways to Add Parentheses

Given a string of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators. The valid operators are +, - and *.

Example 1
Input: “2-1-1”.

((2-1)-1) = 0(2-(1-1)) = 2

Output: [0, 2]

Example 2
Input: “2*3-4*5”

(2*(3-(4*5))) = -34((2*3)-(4*5)) = -14((2*(3-4))*5) = -10(2*((3-4)*5)) = -10(((2*3)-4)*5) = 10

Output: [-34, -14, -10, -10, 10]

class Solution {    public List<Integer> diffWaysToCompute(String input) {        List<Integer> result = new ArrayList<>();        for (int i = 0; i < input.length(); i++) {            if (isOperator(input.charAt(i))) {                char operator = input.charAt(i);                List<Integer> left = diffWaysToCompute(input.substring(0, i));                List<Integer> right = diffWaysToCompute(input.substring(i + 1));                for (int num1 : left) {                    for (int num2 : right) {                        result.add(calculate(num1, num2, operator));                    }                }            }        }        if (result.size() == 0) {            result.add(Integer.valueOf(input));        }        return result;    }    public int calculate(int num1, int num2, char operator) {        int result = 0;        switch(operator) {            case '+' : result = num1 + num2;            break;            case '-' : result = num1 - num2;            break;            case '*' : result = num1 * num2;            break;        }        return result;    }    public boolean isOperator(char c) {        if (c == '+' || c == '-' || c == '*')            return true;        return false;    }}