Here's my python code:

def marsExploration(s):
    sos = "SOS" * (len(s)//3)
    changed = 0
    for index, item in enumerate(s):
        if item != sos[index]:
            changed += 1
    return changed

Herw my java code:

public static int marsExploration(String s) {
 int    count = 0;
char[] ch = s.toCharArray();
for(int i=0; i<s.length(); i+=3) {
    if(ch[i] !='S')
        count++;
    if(ch[i+1] !='O')
        count++;
    if(ch[i+2] !='S')
       count++;
}
return count;
}

Rust best solution

If you’re looking for solutions to the 3-month preparation kit in either Python or Rust, you can find them below: my solutions

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
}

Python best solution

If you’re looking for solutions to the 3-month preparation kit in either Python or Rust, you can find them below: my solutions

def mars_exploration(s):
    # Time complexity: O(n)
    # Space complexity (ignoring input): O(1)
    sos_array = "SOS"
    changed_letters = 0
    for index in range(0, len(s)):
        letter = s[index]
        if letter != sos_array[index % 3]:
            changed_letters += 1

    return changed_letters

//java
    public static int marsExploration(String s) {
    // Write your code here
    int changedLetters = 0;
    char[] chars = s.toCharArray();
    for(int i=0; i<s.length(); i+=3) {
        if(chars[i] != 'S')
            changedLetters++;
        if(chars[i+1] !='O')
            changedLetters++;
        if(chars[i+2] !='S')
            changedLetters++;
    }
    return changedLetters;
}