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> ret = new LinkedList<Integer>(); 
        for (int i = 0; i < input.length(); i++) {
            if (input.charAt(i) == '+' || 
                input.charAt(i) == '-' ||
                input.charAt(i) == '*' ) {
                
                String part1 = input.substring(0, i);
                String part2 = input.substring(i+1);
                //call the function recursively 
                List<Integer> part1Ret = diffWaysToCompute(part1);
                List<Integer> part2Ret = diffWaysToCompute(part2);
                for (Integer p1:part1Ret) {
                    for (Integer p2:part2Ret) {
                        int c = 0; 
                        switch (input.charAt(i)) {
                            case '+': c = p1+p2; break; 
                            case '-': c = p1-p2; break; 
                            case '*': c = p1*p2; break; 
                        }
                        ret.add(c);
                    }
                }
            }
        }
        //if the input is an integer:
        if (ret.size() == 0) 
            ret.add(Integer.valueOf(input)); 
        return ret;
    }
}
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