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 <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 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

<?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
?>

Javascript

<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 code
let 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++

// Efficient C++ program to replace all occurrences
// of "AB" with "C"
#include <bits/stdc++.h>
 
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

<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 code
var str = "helloABworldABGfG";
document.write("The modified string is :" +
               "<br>" + translate(str));
 
// This code is contributed by ipg2016107
 
</script>

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.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready. To complete your preparation from learning a language to DS Algo and many more, please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.

My Personal Notes