input: Write a function to replace all occurrences of a given string with an alternate string. If the target string is not found in the input string, return the original input string.
  assert replace_string("Hello World!", "World", "Python")=="Hello Python!"
  assert replace_string("This is a test", "is", "was")=="Thwas was a test"
  assert replace_string("No change needed", "missing", "found")=="No change needed"
  assert replace_string("", "any", "some")==""
output: def replace_string(input_str, target, alt):
      if target in input_str:
          return input_str.replace(target, alt)
      return input_str