【LeetCode】241. Different Ways to Add Parentheses

unordered_map<string, vector<int> > book;

class Solution { 
public:
    vector<int> diffWaysToCompute(string input) {
        if(book.count(input)) return book[input];
        vector<int> outputs;
        int n = input.length();
        for (int i = 0; i < n; i++) {
            if (input[i] == ‘+‘ || input[i] == ‘-‘ || input[i] == ‘*‘) {
                string left = input.substr(0, i);
                string right = input.substr(i + 1, n - i - 1);
                vector<int> lval = diffWaysToCompute(left);
                vector<int> rval = diffWaysToCompute(right);
                for (int l : lval) {
                    for (int r : rval) {
                        switch (input[i]) {
                            case ‘+‘:
                                outputs.push_back(l + r);
                                break;
                            case ‘-‘:
                                outputs.push_back(l - r);
                                break;
                            default:
                                outputs.push_back(l * r);
                        }
                    } 
                }
            }
        }
        if (outputs.empty())
            outputs.push_back(stoi(input));
        book[input] = outputs;
        return outputs;
    }
};