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

08 Jul, 2022
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++
Java
Python3
C#
PHP
Javascript
// C++ program to replace all occurrences of "AB"// with "C"#include <bits/stdc++.h> 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 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.
# 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# 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// PHP program to replace all// occurrences of "AB" with "C" function translate(&$str){    if ($str[0] == '')        return;     // Start traversing from second character    for ($i = 1; $i < strlen($str); $i++)    {        // If previous character is 'A' and        // current character is 'B"        if ($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 ($j = $i; $j < strlen($str) ; $j++)                $str[$j] = $str[$j + 1];        }    }    return;} // Driver code$str = "helloABworldABGfG";translate($str);echo "The modified string is :\n";echo $str; // This code is contributed// by ChitraNayal?>
<script> // Javascript program to replace all// occurrences of "AB" with "C"function translate(str){         // Start traversing from second character    for(let 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';            let j;                         for(j = i; j < str.length - 1; j++)                str[j] = str[j + 1];                             str[j] = ' ';        }    }    return;} // Driver codelet st = "helloABworldABGfG";let str = st.split("");translate(str); document.write("The modified string is :<br>");document.write(str.join("")); // This code is contributed by avanitrachhadiya2155 </script>
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++
Java
Python3
C#
Javascript
// Efficient C++ program to replace all occurrences// of "AB" with "C"#include <bits/stdc++.h>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 codeint main(){    string str = "helloABworldABGfG";    translate(str);    cout << "The modified string is:" << endl << str;}
// 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.
# 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.
// 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.
<script> // Efficient javascript program to replace// all occurrences of "AB" with "C"function translate(str){    var len = str.length;    if (len < 2)       return;            // Index in modified string    var i = 0;          // Index in original string    var 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;            let firstPart = str.substr(0, i);            let lastPart =  str.substr(i + 1);            str = firstPart + 'C' + lastPart;            i += 1;            continue;        }        let firstPart = str.substr(0, i);        let lastPart =  str.substr(i + 1);        str = firstPart + str[j] + lastPart;        i += 1;        j += 1;    }     if (j == len - 1)    {        let firstPart = str.substr(0, i);        let lastPart =  str.substr(i + 1);        str = firstPart + str[j] + lastPart;        i += 1;    }     // Add a null character to terminate string    let firstPart = str.substr(0, i);    let lastPart =  str.substr(i + 1);    str = firstPart + ' ' + lastPart;    firstPart = str.substr(0, len - 1);    lastPart =  str.substr(len);    str = firstPart + ' ' + lastPart;         // str[len - 1]=' ';    return str;} // Driver codevar str = "helloABworldABGfG";document.write("The modified string is :" +               "<br>" + translate(str)); // This code is contributed by ipg2016107 </script>
The modified string is:
helloCworldCGfG  
Time Complexity : O(n) Auxiliary Space : O(1)
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