Reemplace los espacios en blanco faltantes en una string con el carácter menos frecuente usando Pandas

Vamos a crear un programa en python que reemplazará los espacios 
en blanco en una string con el carácter que aparece en la string al menos 
usando la biblioteca Pandas. 

Ejemplo 1: 

String S = "akash loves gfg" 
here:
'g' comes: 2 times
's' comes: 2 times
'a' comes: 2 times
'h' comes: 1 time 
'o' comes: 1 time 
'k' comes: 1 time 
'v' comes: 1 time 
'e' comes: 1 time 
'f' comes: 1 time 
'l' comes: 1 time 

In this example, there are 7 characters with least frequency 1 so, there can be
7 valid outputs One of the possible output is given below: 
So, the Output String will be: "akashlloveslgfg".

Ejemplo 2: 

string ="goodd noon" 
here:
g comes: 1 time 
o comes: 4 times 
d comes: 2 times 
n comes: 2 times 
So the character with the least frequency 1 is g So here white spaces will be 
replaced by the character g and the output will be: 
"gooddgnoon" 

Ahora, veamos la implementación:

# importing pandas library
import pandas as pd
 
# taking string with white spaces
newstr1 = 'akash loves gfg'
 
# printing the original string
print("Original String given by user:",
      newstr1)
 
# converting string into
# list of characters
ser = pd.Series(list(newstr1))
 
# counting the frequency
# of characters
element_freq = ser.value_counts()
 
# printing character and their
# respective frequency
print(element_freq)
 
current_freq = element_freq.dropna().index[-1]
 
# function element_freq.dropna()
# will  Return a new Series with
# missing values removed
result = "".join(ser.replace(' ',
                             current_freq))
 
print(result)

Producción: