LC 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"
Output: [0, 2]
Explanation: 
((2-1)-1) = 0 
(2-(1-1)) = 2

Example 2:

Input: "2*3-4*5"
Output: [-34, -14, -10, -10, 10]
Explanation: 
(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

Runtime: 4 ms, faster than 77.90% of Java online submissions for Different Ways to Add Parentheses.

class Solution {
  public List<Integer> diffWaysToCompute(String input) {
    List<Integer> ret = new ArrayList<>();
    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);
        List<Integer> part1ret = diffWaysToCompute(part1);
        List<Integer> part2ret = diffWaysToCompute(part2);
        for(int x : part1ret){
          for(int y : part2ret){
            switch (input.charAt(i)) {
              case '+' : ret.add(x + y);
              break;
              case '-' : ret.add(x - y);
              break;
              case '*' : ret.add(x * y);
              break;
            }
          }
        }
      }
    }
    if(ret.size() == 0) ret.add(Integer.valueOf(input));
    return ret;
  }

}