Minimum string (Editorial)

The lexicographically smallest letters can always be inserted into string $a$ (at any position) using the described operation.

Let's combine strings $a$ and $b$. Let s = a + b. Sort the letters of string $s$ in ascending order. The lexicographically smallest letters will be at the beginning of string $s$. If string $a$ contains $k$ letters, the password will be the prefix of string $s$ of length $k$.

Algorithm implementation

Read the input strings $a$ and $b$.

cin >> a;
cin >> b;

Combine strings $a$ and $b$ and sort the resulting string.

s = a + b;
sort(s.begin(), s.end());

Print the answer — the prefix of string $s$ with a length equal to a.size().

cout << s.substr(0, a.size());

Python implementation

Read the input strings $a$ and $b$.

a = input()
b = input()

Combine strings $a$ and $b$.

s = a + b

Sort the string $s$.

s = ''.join(sorted(s))

Print the answer — the prefix of string $s$ with a length equal to $len(a)$.

print(s[:len(a)])