[LeetCode] Expression Add Operators

This post shares a very nice solution, which is rewritten below.
 1 class Solution {
 2 public:
 3     vector<string> addOperators(string num, int target) {
 4         int n = num.length();
 5         if (!n) return {};
 6         vector<string> ans;
 7         for (int i = 1; i <= n; i++) {
 8             string s = num.substr(0, i);
 9             long v = stol(s);
10             if (s != to_string(v)) continue;
11             add(ans, num, target, s, i, v, v, '$');
12         }
13         return ans;
14     }
15 private:
16     void add(vector<string>& ans, string& num, int target, string s, int i, long v, long pv, char op) {
17         int n = num.length();
18         if (i == n && v == target)
19             ans.push_back(s);
20         else {
21             for (int p = i + 1; p <= n; p++) {
22                 string t = num.substr(i, p - i);
23                 long d = stol(t);
24                 if (t != to_string(d)) continue;
25                 add(ans, num, target, s + '+' + t, p, v + d, d, '+');
26                 add(ans, num, target, s + '-' + t, p, v - d, d, '-');
27                 add(ans, num, target, s + '*' + t, p, (op == '+') ? v - pv + pv * d : ((op == '-') ? v + pv - pv * d: pv * d), pv * d, op);
28             }
29         }
30     }
31 };