LeetCode-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]

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

Solution:

public class Solution {
    HashSet<Character> operators;
    
    public List<Integer> diffWaysToCompute(String input) {
        operators = new HashSet<Character>();
        operators.add('+');
        operators.add('-');
        operators.add('*');
        return diffWaysRecur(new StringBuilder().append(input),0,input.length()-1);
    }
    
    // Count different ways to compute sArr[start...end]
    public List<Integer> diffWaysRecur(StringBuilder sArr, int start, int end){
        List<Integer> resList = new ArrayList<Integer>();
        
        // find the first operator.
        int p1 = start;
        while (p1<=end && !operators.contains(sArr.charAt(p1))){
            p1++;
        }
        
        if (p1>end){
            // no operator, return the number.
            resList.add(Integer.parseInt(sArr.substring(start,end+1)));
        }
        
        while (p1<=end){
            // Compute left substring and right substring 
            List<Integer> leftRes = diffWaysRecur(sArr, start, p1-1);
            List<Integer> rightRes = diffWaysRecur(sArr, p1+1, end);
            for (int lVal : leftRes)
                for (int rVal : rightRes){
                    resList.add(compute(sArr.charAt(p1),lVal,rVal));
                }
            
            // Looking for next operator.
            p1++;
            while (p1<=end && !operators.contains(sArr.charAt(p1))){
                p1++;
            }
        }
        
        return resList;
    }
    
    public int compute(char oper, int v1, int v2){
        if (oper == '+') return v1+v2;
        if (oper == '-') return v1-v2;
        if (oper == '*') return v1*v2;
        return 0;
    }
}