Recursively remove all adjacent duplicates
Last Updated :
23 Jul, 2025
Given a string S, the task is to remove all its adjacent duplicate characters recursively.
Examples:
Input: S = "geeksforgeek"
Output: "gksforgk"
Explanation: g(ee)ksforg(ee)k -> gksforgk
Input: S = "abccbccba"
Output: ""
Explanation: ab(cc)b(cc)ba->abbba->a(bbb)a->aa->(aa)->"" (empty string)
[Naive Approach] Using Recursion - O(n ^ 2) Time and O(n ^ 2) Space
The idea is to first iteratively build a new string in result by removing adjacent duplicates. After one full pass, if the length of the string of result remains as original string, returns the result. If changes were made (meaning some duplicates were removed), simply calls itself recursively on the newly formed string. This ensures that any new adjacent duplicates formed by the removal of previous ones are also eliminated. The idea is to gradually "peel off" the duplicates layer by layer until no adjacent duplicates are left.
Below is the implementation of the above approach:
C++
#include <bits/stdc++.h>
using namespace std;
// Function to recursively remove adjacent duplicates
string rremove(string s) {
// Create an empty string to build the result
string sb = "";
// Get the size of the input string
int n = s.size();
// Iterate over the length of current string
for (int i = 0; i < n; i++) {
bool repeated = false;
// Check if the current characteris the same
// as the next one
while (i + 1 < n && s[i] == s[i + 1]) {
repeated = true; // Mark as repeated
// Skip the next character
// since it's a duplicate
i++;
}
// If the character was not repeated,
// add it to the result string
if (!repeated) sb += s[i];
}
// If no changes made, return the result string
if (n == sb.length())
return sb;
// Otherwise, recursively call the function
// to check for more duplicates
return rremove(sb);
}
int main() {
string s = "geeksforgeek";
string result = rremove(s);
cout << result << endl;
return 0;
}
#include <stdio.h>
#include <string.h>
// Function to recursively remove adjacent
// duplicates
void rremove(char *s){
// Get the size of the input string
int n = strlen(s);
// Initialize result string
char sb[101] = "";
int k = 0;
// Iterate over the length of current string
for (int i = 0; i < n; i++){
// Flag to check if the current character
// is repeated
int repeated = 0;
// Check if the current character is the
// same as the next one
while (i + 1 < n && s[i] == s[i + 1]){
// Mark as repeated
repeated = 1;
// Skip the next character since
// it's a duplicate
i++;
}
// If the character was not repeated,
// add it to the result string
if (!repeated)
sb[k++] = s[i];
}
// If no changes made, return the result string
sb[k] = '\0';
if (n == k)
strcpy(s, sb);
else{
// Otherwise, recursively call function
// to check for more duplicates
strcpy(s, sb);
rremove(s);
}
}
int main(){
char s[] = "geeksforgeek";
rremove(s);
printf("%s", s);
return 0;
}
class GfG {
// Function to recursively remove adjacent duplicates
static String rremove(String s)
{
// Create an empty string to build the result
StringBuilder sb = new StringBuilder();
// Get the size of the input string
int n = s.length();
// Iterate over the length of current string
for (int i = 0; i < n; i++) {
// Flag to check if the current
// character is repeated
boolean repeated = false;
// Check if the current character is the same as
// the next one
while (i + 1 < n
&& s.charAt(i) == s.charAt(i + 1)) {
// Mark as repeated
repeated = true;
// Skip the next character since it's a
// duplicate
i++;
}
// If the character was not repeated,
// add it to the result string
if (!repeated)
sb.append(s.charAt(i));
}
// If no changes were made, return the
// result string
if (n == sb.length())
return sb.toString();
// Otherwise, recursively call the function
// to check for more duplicates
return rremove(sb.toString());
}
public static void main(String[] args){
String s = "geeksforgeek";
String result = rremove(s);
System.out.println(result);
}
}
# Function to recursively remove adjacent duplicates
def rremove(s):
# Initialize result string
sb = ""
# Get the size of the input string
n = len(s)
# Iterate over the length of current string
i = 0
while i < n:
# Flag to check if the current character
# is repeated
repeated = False
# Check if the current characte
# r is the same as the next one
while i + 1 < n and s[i] == s[i + 1]:
# Mark as repeated
repeated = True
# Skip the next character since
# it's a duplicate
i += 1
# If the character was not repeated,
# add it to the result string
if not repeated:
sb += s[i]
i += 1
# If no changes were made, return the result
# string
if n == len(sb):
return sb
# Otherwise, recursively call the function \
# to check for more duplicates
return rremove(sb)
s = "geeksforgeek"
result = rremove(s)
print(result)
using System;
class GfG {
// Function to recursively remove adjacent duplicates
static string rremove(string s)
{
// Initialize result string
string sb = "";
// Get the size of the input string
int n = s.Length;
// Iterate over the length of current string
for (int i = 0; i < n; i++) {
// Flag to check if the current character is
// repeated
bool repeated = false;
// Check if the current character is the same as
// the next one
while (i + 1 < n && s[i] == s[i + 1]) {
// Mark as repeated
repeated = true;
// Skip the next character since it's a
// duplicate
i++;
}
// If the character was not repeated, add it to
// the result string
if (!repeated)
sb += s[i];
}
// If no changes were made, return the result string
if (n == sb.Length)
return sb;
// Otherwise, recursively call the function to check
// for more duplicates
return rremove(sb);
}
static void Main(){
string s = "geeksforgeek";
string result = rremove(s);
Console.WriteLine(result);
}
}
// Function to recursively remove adjacent duplicates
function rremove(s)
{
// Initialize result string
let sb = "";
// Get the size of the input string
let n = s.length;
// Iterate over the length of current string
for (let i = 0; i < n; i++) {
// Flag to check if the current character is
// repeated
let repeated = false;
// Check if the current character is the same as the
// next one
while (i + 1 < n && s[i] === s[i + 1]) {
// Mark as repeated
repeated = true;
// Skip the next character since it's a
// duplicate
i++;
}
// If the character was not repeated, add it to the
// result string
if (!repeated)
sb += s[i];
}
// If no changes were made, return the result string
if (n === sb.length)
return sb;
// Otherwise, recursively call the function to check for
// more duplicates
return rremove(sb);
}
let s = "geeksforgeek";
let result = rremove(s);
console.log(result);
Time Complexity: O(n2), In the worst case, the function may need to iterate almost through the entire string for each recursion, which makes the time complexity as O(N2).
Auxiliary Space: O(n2), as we are storing the new string in each recursive call and there can be n recursive call in the worst case.
[Expected Approach] Using Recursion - O(n2) Time and O(n) Space:
This solution also removes adjacent duplicates but it does this by modifying the string in place. It uses an index to track where to place non-duplicate characters. It skips over duplicate characters and moves only unique characters forward. After processing the original string, trims the original string to remove extra characters. If any adjacent duplicates were removed in this process then recursively call the function itself to repeat this process again. This approach is efficient because it modifies the string directly without creating new ones.
Below is the implementation of above approach.
C++
#include <bits/stdc++.h>
using namespace std;
// Helper function to remove adjacent duplicates
void remove_util(string &str, int n) {
// Get the length of the string
int len = str.length();
// Index to store the result string
int k = 0;
// Iterate over the string to remove adjacent
// duplicates
for (int i = 0; i < n; i++) {
// Check if the current character is the same
// as the next one
if (i < n - 1 && str[i] == str[i + 1]) {
// Skip all the adjacent duplicates
while (i < n - 1 && str[i] == str[i + 1]) {
i++;
}
} else {
// If not a duplicate, store the character
str[k++] = str[i];
}
}
// Remove the remaining characters from the
// original string
str.erase(k);
// If any adjacent duplicates were removed,
// recursively check for more
if (k != n)
remove_util(str, k);
}
// Function to initiate the removal of adjacent
// duplicates
string rremove(string s) {
// Call the helper function
remove_util(s, s.length());
// Return the modified string
return s;
}
int main() {
string s = "geeksforgeek";
cout << rremove(s) << endl;
return 0;
}
#include <stdio.h>
#include <string.h>
// Helper function to remove adjacent duplicates
void remove_util(char* str, int n) {
// Get the length of the string
int len = strlen(str);
// Index to store the result string
int k = 0;
// Iterate over the string to remove adjacent
// duplicates
for (int i = 0; i < n; i++) {
// Check if the current character
// is the same as the next one
if (i < n - 1 && str[i] == str[i + 1]) {
// Skip all the adjacent duplicates
while (i < n - 1 && str[i] == str[i + 1]) {
i++;
}
} else {
// If not a duplicate, store the character
str[k++] = str[i];
}
}
// Remove the remaining characters from the
// original string
str[k] = '\0';
// If any adjacent duplicates were removed,
// recursively check for more
if (k != n)
remove_util(str, k);
}
// Function to initiate the removal of adjacent
// duplicates
void rremove(char* s) {
// Call the helper function
remove_util(s, strlen(s));
}
int main() {
char s[] = "geeksforgeek";
rremove(s);
printf("%s\n", s);
return 0;
}
public class GfG {
// Helper function to remove adjacent duplicates
static void removeUtil(StringBuilder str, int n){
// Index to store the result string
int k = 0;
// Iterate over the string to remove adjacent
// duplicates
for (int i = 0; i < n; i++) {
// Check if the current character is the same as
// the next one
if (i < n - 1
&& str.charAt(i) == str.charAt(i + 1)) {
// Skip all the adjacent duplicates
while (i < n - 1
&& str.charAt(i)
== str.charAt(i + 1)) {
i++;
}
}
else {
// If not a duplicate, store the character
str.setCharAt(k++, str.charAt(i));
}
}
// Remove the remaining characters from the original
// string
str.setLength(k);
// If any adjacent duplicates were removed,
// recursively check for more
if (k != n)
removeUtil(str, k);
}
// Function to initiate the removal of adjacent
// duplicates
static String rremove(String s){
StringBuilder str = new StringBuilder(s);
// Call the helper function
removeUtil(str, str.length());
// Return the modified string
return str.toString();
}
public static void main(String[] args){
String s = "geeksforgeek";
System.out.println(rremove(s));
}
}
# Helper function to remove adjacent duplicates
def remove_util(s, n):
# Index to store the result string
k = 0
# Iterate over the string to remove adjacent
# duplicates
i = 0
while i < n:
# Check if the current character is the
# same as the next one
if i < n - 1 and s[i] == s[i + 1]:
# Skip all the adjacent duplicates
while i < n - 1 and s[i] == s[i + 1]:
i += 1
else:
# If not a duplicate, store the character
s[k] = s[i]
k += 1
i += 1
# Remove the remaining characters from
# the original string
s = s[:k]
# If any adjacent duplicates were removed,
# recursively check for more
if k != n:
s = remove_util(list(s), k)
return s
# Function to initiate the removal of adjacent duplicates
def rremove(s):
# Convert the string to a list to allow modification
s = list(s)
# Call the helper function
return ''.join(remove_util(s, len(s)))
# Example usage
s = "geeksforgeek"
print(rremove(s))
using System;
using System.Text;
class GfG {
// Helper function to remove adjacent duplicates
static void RemoveUtil(StringBuilder str, int n)
{
// Index to store the result string
int k = 0;
// Iterate over the string to remove adjacent
// duplicates
for (int i = 0; i < n; i++) {
// Check if the current character
// is the same as the next one
if (i < n - 1 && str[i] == str[i + 1]) {
// Skip all the adjacent duplicates
while (i < n - 1
&& str[i] == str[i + 1]) {
i++;
}
}
else {
// If not a duplicate, store the character
str[k++] = str[i];
}
}
// Remove the remaining characters from
// the original string
str.Length = k;
// If any adjacent duplicates were removed,
// recursively check for more
if (k != n)
RemoveUtil(str, k);
}
// Function to initiate the removal of adjacent
// duplicates
static string RRemove(string s)
{
StringBuilder str = new StringBuilder(s);
// Call the helper function
RemoveUtil(str, str.Length);
// Return the modified string
return str.ToString();
}
static void Main()
{
string s = "geeksforgeek";
Console.WriteLine(RRemove(s));
}
}
// Helper function to remove adjacent duplicates
function removeUtil(str, n) {
let k = 0;
let result = [];
// Iterate over the string to remove adjacent
// duplicates
for (let i = 0; i < n; i++) {
// Check if the current character is the
// same as the next one
if (i < n - 1 && str[i] === str[i + 1]) {
// Skip all the adjacent duplicates
while (i < n - 1 && str[i] === str[i + 1]) {
i++;
}
} else {
// If not a duplicate, store the character
result[k++] = str[i];
}
}
// If any adjacent duplicates were removed,
// recursively check for more
if (k !== n) {
return removeUtil(result.join(''), k);
}
return result.join('');
}
// Function to initiate the removal of adjacent
// duplicates
function rremove(s) {
// Call the helper function
return removeUtil(s, s.length);
}
let s = "geeksforgeek";
console.log(rremove(s));
Time Complexity: O(n2), In the worst case, the function may need to iterate almost through the entire string for each recursion, which makes the time complexity as O(n2).
Auxiliary Space: O(n), considering the recursive call stack.
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