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

• Difficulty Level : Easy
• Last Updated : 02 Jun, 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 more all characters one position back.

## C++

 `// C++ program to replace all occurrences of "AB"``// with "C"``#include ` `void` `translate(``char``* str)``{``    ``if` `(str == ``''``)``        ``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 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.`

## PHP

 ``

## 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 code``int` `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 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)

This article is contributed by Roshni Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.