Possible combinations of string in python

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A permutation also called an “arrangement number” or “order,” is a rearrangement of the elements of an ordered list S into a one-to-one correspondence with S itself. A string of length n has n! permutation. 

Source: Mathword(http://mathworld.wolfram.com/Permutation.html)

Below are the permutations of string ABC. 
ABC ACB BAC BCA CBA CAB

Here is a solution that is used as a basis in backtracking.

Python3

def toString(List):

    return ''.join(List)

def permute(a, l, r):

    if l == r:

        print (toString(a))

    else:

        for i in range(l, r + 1):

            a[l], a[i] = a[i], a[l]

            permute(a, l + 1, r)

            a[l], a[i] = a[i], a[l]

string = "ABC"

n = len(string)

a = list(string)

permute(a, 0, n-1)

Output: 

ABC
ACB
BAC
BCA
CBA
CAB

Algorithm Paradigm: Backtracking 

Time Complexity: O(n*n!) Note that there are n! permutations and it requires O(n) time to print a permutation.

Auxiliary Space: O(r – l)

Note: The above solution prints duplicate permutations if there are repeating characters in the input string. Please see the below link for a solution that prints only distinct permutations even if there are duplicates in input.
Print all distinct permutations of a given string with duplicates. 
Permutations of a given string using STL

Another approach:

Python3

def permute(s, answer):

    if (len(s) == 0):

        print(answer, end = "  ")

        return

    for i in range(len(s)):

        ch = s[i]

        left_substr = s[0:i]

        right_substr = s[i + 1:]

        rest = left_substr + right_substr

        permute(rest, answer + ch)

answer = ""

s = input("Enter the string : ")

print("All possible strings are : ")

permute(s, answer)

Output:

Enter the string : abc
All possible strings are : abc  acb  bac  bca  cab  cba

Time Complexity: O(n*n!) The time complexity is the same as the above approach, i.e. there are n! permutations and it requires O(n) time to print a permutation.

Auxiliary Space: O(|s|)