pub fn mars_exploration(s: &str) -> i32 {
    //Time complexity: O(n)
    //Space complexity (ignoring input): O(1)
    let sos_array: Vec<_> = "SOS".chars().collect();
    let mut changed_letters = 0;
    for (index, letter) in s.char_indices(){
        if letter != sos_array[index%3]{
            changed_letters += 1;
        }
    }
    changed_letters
}