241. Different Ways to Add Parentheses

Given a string expression of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators. You may return the answer in any order.

class Solution {
    unordered_map<string, vector<int>> dp;
public:
    vector<int> diffWaysToCompute(string& exp) {
        if(dp.count(exp)) return dp[exp];
        vector<int>& res = dp[exp];
        for(int i = 0; i < exp.length(); i++) {
            if(!isdigit(exp[i])) {
                string left = exp.substr(0,i);
                string right = exp.substr(i + 1);
                vector<int> leftResult = diffWaysToCompute(left);
                vector<int> rightResult = diffWaysToCompute(right);
                for(int l : leftResult) {
                    for(int r : rightResult) {
                        if(exp[i] == '+') {
                            res.push_back(l + r);
                        } else if(exp[i] == '-') {
                            res.push_back(l - r);
                        } else {
                            res.push_back(l * r);
                        }
                    }
                }
            }
        }
        
        if(res.empty()) {
            res.push_back(stoi(exp));
        }
        return res;
    }
};