1055

class Solution {
public:
    int shortestWay(string source, string target) {
        int slen = source.length(), tlen = target.length();
        int ans=0, i=0, pos=-1;
        while(i<tlen && ans<=i){
            pos = source.find(target[i], pos+1);
            if(pos == string::npos){
                ans++;
                pos = source.find(target[i]);
                if(pos == string::npos) return -1;
            }
            i++;
        }
        return ans+1;
    }
};

---------------------

class Solution {
public:
    int shortestWay(string s, string t, int res = 0) {
      vector<int> ch_ids[26] = {};
      for (auto j = 0; j < s.size(); ++j) ch_ids[s[j] - 'a'].push_back(j);
      for (auto i = 0, j = -1; i < t.size(); ) {
        auto &ids = ch_ids[t[i] - 'a'];
        if (ids.empty()) return -1;
        auto it = upper_bound(begin(ids), end(ids), j);
        if(it == end(ids)) ++res, j = -1;
        else ++i, j = *it;
      }
      return res + 1;
    }
};