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Python Program To Recursively Remove All Adjacent Duplicates

  • Last Updated : 23 Dec, 2021

Given a string, recursively remove adjacent duplicate characters from the string. The output string should not have any adjacent duplicates. See the following examples.


Input: azxxzy 
Output: ay 
First “azxxzy” is reduced to “azzy”. 
The string “azzy” contains duplicates, 
so it is further reduced to “ay”.

Input: geeksforgeeg 
Output: gksfor 
First “geeksforgeeg” is reduced to 
“gksforgg”. The string “gksforgg” 
contains duplicates, so it is further 
reduced to “gksfor”.

Input: caaabbbaacdddd 
Output: Empty String

Input: acaaabbbacdddd 
Output: acac 

The following approach can be followed to remove duplicates in O(N) time: 

  • Start from the leftmost character and remove duplicates at left corner if there are any.
  • The first character must be different from its adjacent now. Recur for string of length n-1 (string without first character).
  • Let the string obtained after reducing right substring of length n-1 be rem_str. There are three possible cases 
    1. If first character of rem_str matches with the first character of original string, remove the first character from rem_str.
    2. If remaining string becomes empty and last removed character is same as first character of original string. Return empty string.
    3. Else, append the first character of the original string at the beginning of rem_str.
  • Return rem_str.

Below image is a dry run of the above approach:

Below is the implementation of the above approach:


# Python program to remove all 
# adjacent duplicates from a string
# Recursively removes adjacent 
# duplicates from str and returns
# new string. las_removed is a 
# pointer to last_removed character
def removeUtil(string, last_removed):
    # If length of string is 1 or 0
    if len(string) == 0 or len(string) == 1:
        return string
    # Remove leftmost same characters 
    # and recur for remaining 
    # string
    if string[0] == string[1]:
        last_removed = ord(string[0])
        while len(string) > 1 and 
                  string[0] == string[1]:
            string = string[1:]
        string = string[1:]
        return removeUtil(string, last_removed)
    # At this point, the first 
    # character is definiotely different
    # from its adjacent. Ignore first 
    # character and recursively 
    # remove characters from remaining string
    rem_str = removeUtil(string[1:], 
    # Check if the first character 
    # of the rem_string matches 
    # with the first character of 
    # the original string
    if len(rem_str) != 0 and 
           rem_str[0] == string[0]:
        last_removed = ord(string[0])
        return (rem_str[1:])
    # If remaining string becomes 
    # empty and last removed character
    # is same as first character of 
    # original string. This is needed
    # for a string like "acbbcddc"
    if len(rem_str) == 0 and 
       last_removed == ord(string[0]):
        return rem_str
    # If the two first characters of 
    # str and rem_str don't match, 
    # append first character of str 
    # before the first character of 
    # rem_str.
    return ([string[0]] + rem_str)
def remove(string):
    last_removed = 0
    return toString(removeUtil(toList(string), 
# Utility functions
def toList(string):
    x = []
    for i in string:
    return x
def toString(x):
    return ''.join(x)
# Driver program
string1 = "geeksforgeeg"
print remove(string1)
string2 = "azxxxzy"
print remove(string2)
string3 = "caaabbbaac"
print remove(string3)
string4 = "gghhg"
print remove(string4)
string5 = "aaaacddddcappp"
print remove(string5)
string6 = "aaaaaaaaaa"
print remove(string6)
string7 = "qpaaaaadaaaaadprq"
print remove(string7)
string8 = "acaaabbbacdddd"
print remove(string8)
string9 = "acbbcddc"
print remove(string9)
# This code is contributed by BHAVYA JAIN



Time Complexity: The time complexity of the solution can be written as T(n) = T(n-k) + O(k) where n is length of the input string and k is the number of first characters which are same. Solution of the recurrence is O(n)

Thanks to Prachi Bodke for suggesting this problem and initial solution. 

Please refer complete article on Recursively remove all adjacent duplicates for more details!

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