Define the function `is_permutation` that returns true if the string `s1` is a permutation of `s2`, otherwise it returns false `s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2` and all the characters in `s1` appear in `s2` even if in different order)
Define the **function**`is_permutation` that returns true if:
- the string `s1` is a permutation of `s2`, otherwise it returns false.
- `s1` is a permutation of `s2` if all the elements in `s1` appear the same number of times in `s2` and all the characters in `s1` appear in `s2` even if they are in different order.