Generate a list using given frequency list

Given a list, the task is to generate another list containing numbers from 0 to n, from given list whose each element is frequency of numbers in the generated list.

Input: freq = [1, 4, 3, 5]
Output: [0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Explanation:
Number = 0
Input[0] = 1 Output = [0]

Number =1
Input[1] = 4 Output = [0, 1, 1, 1, 1] (repeats 1 four times)

Number =2
Input[2] = 3  Output = [0, 1, 1, 1, 1, 2, 2, 2] (repeats 2 three times)

Number =3
Input[3] = 5  Output = [0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3] (repeats 3 five times)

Below are some ways to achieve above task.

Method #1 : Using Iteration

# Python code to generate list containing numbers from 0 to 'n'
# having frequency of no from another list

# List initialisation
Input = [1, 4, 3, 5]
Output = []

# Number initialisation
no = 0

# using iteration
for rep in Input:
    for elem in range(rep):
        Output.append(no)
    no += 1

# printing output
print(Output)

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Method #2 : Using enumerate and list comprehension

# Python code to generate list containing numbers from 0 to 'n'
# having frequency of no from another list

# List initialisation
Input = [1, 4, 3, 5]

# Using enumerate and list comprehension
Output = [no for no, rep in enumerate(Input)
                     for elem in range(rep)]

# Printing output
print(Output)

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]

Method #3 : Using enumerate and chain

# Python code to generate list containing numbers from 0 to 'n'
# having frequency of no from another list

# List initialisation
Input = [1, 4, 3, 5]

# Importing
from itertools import repeat, chain

# Using chain and enumerate
Output = list(chain.from_iterable((repeat(x, y))
                  for x, y in enumerate(Input)))

# Printing output
print(Output)

Output:

[0, 1, 1, 1, 1, 2, 2, 2, 3, 3, 3, 3, 3]