To check which all characters from first string occur in the second string, we can simply traverse are first string, and for each character verify whether it is present in second string or not.

Initialize an answer string and iterate over the first string.
For every character, iterate in second string and check if it is present or not.
If the character is present, skip that character. If it is not present, add it to our answer string.
Return the answer string once the first string is traversed completely.

Code

#include <bits/stdc++.h>
using namespace std;

// Class for solution
class Solution {
public:
    // Brute force function to remove characters
    string removeChars(string str1, string str2) {
        string result = "";

        // Traverse str1
        for (int i = 0; i < str1.length(); i++) {
            bool found = false;

            // For each character in str1, check in str2
            for (int j = 0; j < str2.length(); j++) {
                if (str1[i] == str2[j]) {
                    found = true;
                    break;
                }
            }

            // If not found, add to result
            if (!found) {
                result += str1[i];
            }
        }

        return result;
    }
};

int main() {
    string str1 = "computer";
    string str2 = "cat";

    Solution sol;

    string result = sol.removeChars(str1, str2);

    cout << "Result: " << result << endl;

    return 0;
}