[LeetCode] Expression Add Operators

This post shares a very nice solution, which is rewritten below.

class Solution {
public:
    vector<string> addOperators(string num, int target) {
        int n = num.length();
        if (!n) return {};
        vector<string> ans;
        for (int i = 1; i <= n; i++) {
            string s = num.substr(0, i);
            long v = stol(s);
            if (s != to_string(v)) continue;
            add(ans, num, target, s, i, v, v, '$');
        }
        return ans;
    }
private:
    void add(vector<string>& ans, string& num, int target, string s, int i, long v, long pv, char op) {
        int n = num.length();
        if (i == n && v == target)
            ans.push_back(s);
        else {
            for (int p = i + 1; p <= n; p++) {
                string t = num.substr(i, p - i);
                long d = stol(t);
                if (t != to_string(d)) continue;
                add(ans, num, target, s + '+' + t, p, v + d, d, '+');
                add(ans, num, target, s + '-' + t, p, v - d, d, '-');
                add(ans, num, target, s + '*' + t, p, (op == '+') ? v - pv + pv * d : ((op == '-') ? v + pv - pv * d: pv * d), pv * d, op);
            }
        }
    }
};