03. Stream Of Letters

#include <iostream>

using namespace std;

int main() {
    string input, word, message;
    cin >> input;

    int  c = 0, o = 0, n = 0;

    while (input != "End") {
        char letter = input[0];

        if (letter >= 'A' && letter <= 'Z' || letter >= 'a' && letter <= 'z') {
            if (letter == 'c' && c == 0) {
                c++;
            }
            else if (letter == 'o' && o == 0) {
                o++;
            }
            else if (letter == 'n' && n == 0) {
                n++;
            }
            else {
                word += letter;
            }

            if (c == 1 && o == 1 && n == 1) {
                c--; o--; n--;
                message += " " + word;
                word = "";
            }
        }

        cin >> input;
    }

    cout << message << endl;
    return 0;
}


Решение с find(), replace() и isalpha():

#include <iostream>
#include <string>

using namespace std;

int main() {
    string input, word, message, secretWord = "con";
    cin >> input;

    while (input != "End") {
        char letter = input[0];

        if (secretWord.find(letter) != -1) {
            secretWord.replace(secretWord.find(letter), 1, "");

            if (secretWord == "") {
                secretWord = "con";
                message += word + " ";
                word = "";
            }
        }
        else if (isalpha(letter)) {
            word += letter;
        }

        cin >> input;
    }

    cout << message << endl;
    return 0;
}