【PAT (Advanced Level) Practice】1050 String Subtraction (20 分)

Given two strings $S_1$ and $S_2$, $S=S_1−S_2$ is defined to be the remaining string after taking all the characters in $S_2$ from $S_1$. Your task is simply to calculate $S_1−S_2$ for any given strings. However, it might not be that simple to do it fast.

Input Specification:
Each input file contains one test case. Each case consists of two lines which gives $S_1$ and $S_2$, respectively. The string lengths of both strings are no more than $10^4$. It is guaranteed that all the characters are visible ASCII codes and white space, and a new line character signals the end of a string.

Output Specification:
For each test case, print $S_1−S_2$ in one line.

Sample Input:

They are students.
aeiou

Sample Output:

Thy r stdnts.

---

#include<iostream>
#include<unordered_set>

using namespace std;

int main(){
    
    string s1, s2;
    
    getline(cin, s1);
    getline(cin, s2);
    
    unordered_set<char> hash;
    for(auto c: s2) hash.insert(c);
    
    for(auto c: s1)
        if(!hash.count(c))
            cout << c;
    
    cout << endl;
    
    return 0;   
}