19. The lock code is a seven-digit number, the first three digits of which are the same, and the last four digits are also the same. The sum of all the digits of this number is a two-digit number, the first digit of which matches the first digit of the code, and the last digit matches the last digit of the code. Find this code.
19. Let the cipher be $\overline{a a b b b b b}$, then by the condition $3 a+4 b=10 a+b$, from which $7 a=3 b$. Since $a$ and $b$ are digits, the equality is possible only when $a=3$ and $b=7$. Therefore, the cipher is 3337777. (The notation $\bar{a} a b b b b$ represents a seven-digit number, where $b$ is the unit of the first place, $b$ is the unit of the second place, ..., $a$ is the unit of the fifth place, ..., $a$ is the unit of the seventh place.)