03. Stream Of Letters

word = ""
secret = "con"
command = input()

while command != "End":
    if secret.find(command) != -1:
        secret = secret.replace(command, "")
        if len(secret) == 0:
            print(word, end=" ")
            secret = "con"
            word = ""
    elif command.isalpha():
        word += command
    command = input()

Или:

secret = 'con'
word = ''
symbol = input()
while symbol != 'End':
    if symbol in secret:
        secret = secret.replace(symbol, '')
        if secret == '':
            secret = 'con'
            print(word, end=' ')
            word = ''
    elif symbol.isalpha():
        word += symbol
    symbol = input()

Или:

secret = 'con'
string = ''
word = ''
symbol = input()
while symbol != 'End':
    if symbol in secret:
        secret = secret.replace(symbol, '')
        if secret == '':
            secret = 'con'
            string += word + ' '
            word = ''
    elif symbol.isalpha():
        word += symbol
    symbol = input()
print(string)