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]

https://leetcode.com/discuss/48488/c-4ms-recursive-method

class Solution {
public:
    vector<int> diffWaysToCompute(string input) {
        vector<int> ret;
        for(int i = 0; i < input.length(); ++i)
            if(input[i] == '+' || input[i] == '-' || input[i] == '*'){
                vector<int> r1, r2;
                string s1 = input.substr(0, i);
                string s2 = input.substr(i + 1);
                if(Map.find(s1) != Map.end()){
                    r1 = Map[s1];
                }
                else{
                    r1 = diffWaysToCompute(s1);
                }
                
                if(Map.find(s2) != Map.end()){
                    r2 = Map[s2];
                }
                else{
                    r2 = diffWaysToCompute(s2);
                }
                
                for(auto x : r1)
                for(auto y : r2)
                switch(input[i]){
                    case '+': ret.push_back(x + y); break;
                    case '-': ret.push_back(x - y); break;
                    case '*': ret.push_back(x * y); break;
                }
            }
        if(ret.empty()){
            ret.push_back(atoi(input.c_str()));
        }
        Map[input] = ret;
        return ret;
    }
private:
    unordered_map<string, vector<int>> Map;
};