Reverse a number

You are given a natural number $n$. Find the number obtained by writing its digits in reverse order.

A single integer $n$ ($1 \le n \le 10^{18}$).

A single integer. Leading zeros are not written: for $1200$ the answer is $21$.

Build the new number digit by digit: on each step $r = r \cdot 10 + n \bmod 10$, then integer-divide $n$ by $10$. Leading zeros then disappear by themselves.