# Replace all occurrences of string AB with C without using extra space

Given a string str that may contain one more occurrences of “AB”. Replace all occurrences of “AB” with “C” in str.

Examples:

Input  : str = "helloABworld"
Output : str = "helloCworld"

Input  : str = "fghABsdfABysu"
Output : str = "fghCsdfCysu"

A simple solution is to find all occurrences of “AB”. For every occurrence, replace it with C and move all characters one position back.

Implementation:

## C++

 // C++ program to replace all occurrences of "AB" // with "C" #include   void translate(char* str) {     if (str[0] == '\0')         return;       // Start traversing from second character     for (int i=1; str[i] != '\0'; i++)     {         // If previous character is 'A' and         // current character is 'B"         if (str[i-1]=='A' && str[i]=='B')         {             // Replace previous character with             // 'C' and move all subsequent             // characters one position back             str[i-1] = 'C';             for (int j=i; str[j]!='\0'; j++)                 str[j] = str[j+1];         }     }     return; }   // Driver code int main() {     char str[] = "helloABworldABGfG";     translate(str);     printf("The modified string is :\n");     printf("%s", str); }

## Java

 // Java program to replace all // occurrences of "AB" with "C" import java.io.*;   class GFG {           static void translate(char str[])     {         // Start traversing from second character         for (int i = 1; i < str.length; i++)         {             // If previous character is 'A' and             // current character is 'B"             if (str[i - 1] == 'A' && str[i] == 'B')             {                 // Replace previous character with                 // 'C' and move all subsequent                 // characters one position back                 str[i - 1] = 'C';                 int j;                 for (j = i; j < str.length - 1; j++)                     str[j] = str[j + 1];                 str[j] = ' ';                               }         }         return;     }           // Driver code     public static void main(String args[])     {         String st = "helloABworldABGfG";         char str[] = st.toCharArray();         translate(str);         System.out.println("The modified string is :");         System.out.println(str);     } }     // This code is contributed by Nikita Tiwari.

## Python3

 # Python 3 program to replace all # occurrences of "AB" with "C"   def translate(st) :           # Start traversing from second character     for i in range(1, len(st)) :                   # If previous character is 'A'         # and current character is 'B"         if (st[i - 1] == 'A' and st[i] == 'B') :                           # Replace previous character with             # 'C' and move all subsequent             # characters one position back             st[i - 1] = 'C'                           for j in range(i, len(st) - 1) :                 st[j] = st[j + 1]                               st[len(st) - 1] = ' '                       return   # Driver code st = list("helloABworldABGfG") translate(st)   print("The modified string is :") print(''.join(st))   # This code is contributed by Nikita Tiwari.

## C#

 // C# program to replace all // occurrences of "AB" with "C" using System;   class GFG {           static void translate(char []str)     {                   // Start traversing from second character         for (int i = 1; i < str.Length; i++)         {             // If previous character is 'A' and             // current character is 'B"             if (str[i - 1] == 'A' && str[i] == 'B')             {                 // Replace previous character with                 // 'C' and move all subsequent                 // characters one position back                 str[i - 1] = 'C';                 int j;                 for (j = i; j < str.Length - 1; j++)                     str[j] = str[j + 1];                 str[j] = ' ';                               }         }         return;     }           // Driver code     public static void Main()     {         String st = "helloABworldABGfG";         char []str = st.ToCharArray();         translate(str);         Console.WriteLine("The modified string is :");         Console.Write(str);     } }     // This code is contributed by Nitin Mittal.



## Javascript



Output :

The modified string is :
helloCworldCGfG

Time Complexity : O(n2
Auxiliary Space : O(1)

An efficient solution is to keep track of two indexes, one for modified string (i in below code) and other for original string (j in below code). If we find “AB” at current index j, we increment j by 2 and i by 1. Otherwise, we increment both and copy character from j to i.

Below is implementation of above idea.

## C++

 // Efficient C++ program to replace all occurrences // of "AB" with "C" #include using namespace std;   void translate(string &str) {     int len = str.size();     if (len < 2)         return;       // Index in modified string     int i = 0;       // Index in original string     int j = 0;       // Traverse string     while (j < len - 1) {         // Replace occurrence of "AB" with "C"         if (str[j] == 'A' && str[j + 1] == 'B') {             // Increment j by 2             j = j + 2;             str[i++] = 'C';             continue;         }         str[i++] = str[j++];     }       if (j == len - 1)         str[i++] = str[j];       // add a null character to terminate string     str[i] = ' ';     str[len - 1] = ' '; }   // Driver code int main() {     string str = "helloABworldABGfG";     translate(str);     cout << "The modified string is:" << endl << str; }

## Java

 // Efficient Java program to replace // all occurrences of "AB" with "C" import java.io.*;   class GFG {           static void translate(char str[])     {         int len = str.length;         if (len < 2)         return;               // Index in modified string         int i = 0;                   // Index in original string         int j = 0;               // Traverse string         while (j < len - 1)         {             // Replace occurrence of "AB" with "C"             if (str[j] == 'A' && str[j + 1] == 'B')             {                 // Increment j by 2                 j = j + 2;                 str[i++] = 'C';                 continue;             }             str[i++] = str[j++];         }               if (j == len - 1)         str[i++] = str[j];               // add a null character to terminate string         str[i] = ' ';         str[len - 1]=' ';     }           // Driver code     public static void main(String args[])     {         String st="helloABworldABGfG";         char str[] = st.toCharArray();         translate(str);         System.out.println("The modified string is :");         System.out.println(str);     } }     // This code is contributed // by Nikita Tiwari.

## Python3

 # Python 3 program to replace all # occurrences of "AB" with "C"   def translate(st) :     l = len(st)           if (l < 2) :         return       i = 0 # Index in modified string     j = 0 # Index in original string       # Traverse string     while (j < l - 1) :                   # Replace occurrence of "AB" with "C"         if (st[j] == 'A' and st[j + 1] == 'B') :                           # Increment j by 2             j += 2             st[i] = 'C'             i += 1             continue                   st[i] = st[j]         i += 1         j += 1             if (j == l - 1) :         st[i] = st[j]         i += 1       # add a null character to     # terminate string     st[i] = ' '     st[l-1] = ' '   # Driver code st = list("helloABworldABGfG") translate(st)   print("The modified string is :") print(''.join(st))   # This code is contributed by Nikita Tiwari.

## C#

 // Efficient C# program to replace // all occurrences of "AB" with "C" using System;   class GFG {           static void translate(char []str)     {         int len = str.Length;         if (len < 2)             return;               // Index in modified string         int i = 0;                   // Index in original string         int j = 0;               // Traverse string         while (j < len - 1)         {                           // Replace occurrence of "AB" with "C"             if (str[j] == 'A' && str[j + 1] == 'B')             {                 // Increment j by 2                 j = j + 2;                 str[i++] = 'C';                 continue;             }             str[i++] = str[j++];         }               if (j == len - 1)             str[i++] = str[j];               // add a null character to         // terminate string         str[i] = ' ';         str[len - 1]=' ';     }           // Driver code     public static void Main()     {         String st="helloABworldABGfG";         char []str = st.ToCharArray();         translate(str);         Console.Write("The modified string is :");         Console.Write(str);     } }   // This code is contributed by nitin mittal.

## Javascript



Output

The modified string is:
helloCworldCGfG

Time Complexity : O(n)
Auxiliary Space : O(1)

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