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

• Difficulty Level : Easy
• Last Updated : 06 Oct, 2021

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.

## C++

 // C++ program to replace all occurrences of "AB"// with "C"#include  void translate(char* str){    if (str[0] == '')        return;     // Start traversing from second character    for (int i=1; str[i] != ''; 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]!=''; j++)                str[j] = str[j+1];        }    }    return;} // Driver codeint 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 codest = 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  void translate(char* str){    int len = strlen(str);    if (len < 2)       return;     int i = 0;  // Index in modified string    int j = 0; // Index in original string     // 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] = '';} // Driver codeint main(){    char str[] = "helloABworldABGfG";    translate(str);    printf("The modified string is :\n");    printf("%s", 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 codest = 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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