1055. Shortest Way to Form String

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., “ace” is a subsequence of “abcde” while “aec” is not).

Given two strings source and target, return the minimum number of subsequences of source such that their concatenation equals target. If the task is impossible, return -1

• Time : O(nlogn)
• Space : O(n)

class Solution {
public:
    int shortestWay(string source, string target) {
        unordered_map<char, set<int>> seq;
        for(int i = 0; i < source.length(); i++) {
            seq[source[i]].insert(i);
        }
        int res = 0, mi = -1;
        for(auto& t : target) {
            if(!seq.count(t)) return -1; //not contain character case
            
            auto it = seq[t].upper_bound(mi);
            if(it == seq[t].end()) {
                res++;
                mi = *seq[t].begin();
            } else mi = *it;
        }
        
        return res + (mi >= 0);
    }
};