天池 在线编程 Character deletion

Example 1:
Input:  str=”They are students”，sub=”aeiou”
Output: ”Thy r stdnts”

class Solution {
public:
    /**
     * @param str: The first string given
     * @param sub: The given second string
     * @return: Returns the deleted string
     */
    string CharacterDeletion(string &str, string &sub) {
        // write your code here
        unordered_set<char> s(sub.begin(), sub.end());
        string ans;
        for(char c : str)
        {
            if(!s.count(c))
                ans += c;
        }
        return ans;
    }
};