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]

public class Solution {    public List<Integer> diffWaysToCompute(String input) {        List<Integer> res = new ArrayList<Integer>();        for (int i = 0; i < input.length(); i++) {            char c = input.charAt(i);            if (c == '-' || c == '+' || c == '*') {                String left = input.substring(0, i);                String right = input.substring(i+1);                List<Integer> al = diffWaysToCompute(left);                List<Integer> ar = diffWaysToCompute(right);                for (int l : al) {                    for (int r : ar) {                        if (c == '-') res.add(l - r);                        if (c == '+') res.add(l + r);                        if (c == '*') res.add(l * r);                    }                }            }        }        if (res.size() == 0) res.add(Integer.valueOf(input));        return res;    }}