input: Write a function to reverse a string by returning the characters of the string in reverse order. Implement the function using a for loop that iterates from the end of the string to the beginning. The function should take a single string argument and return the reversed string.
  assert reverse_string("Hello")=="olleH"
  assert reverse_string("Python")=="nohtyP"
  assert reverse_string("12345")=="54321"
  assert reverse_string("")==""
output: def reverse_string(s):
      reversed_str = ""
      for i in range(len(s)-1, -1, -1):
          reversed_str += s[i]
      return reversed_str