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"

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 
?> 

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. 

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 contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

PHP

C++

Java

Python3

C#

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