Q:

# Pair Case: C++ program to enter \'*\' between two identical characters in a string

Given a string, compute recursively a new string where identical chars that are adjacent in the original string are separated from each other by a "*".

Sample Input 1: "hello"

Sample Output 1: "hel*lo"

Sample Input 2: "xxyy"

Sample Output 2: "x*xy*y"

Sample Input 3: "aaaa"

Sample Output 3: "a*a*a*a"

Explanation:

In this question, we have to add a star between any pair of same letters. This could be easily achieved using recursion. When the start index is equal to (start + 1) index, we will shift all the letters from (start + 1) by 1 on the right side and on (start + 1), we will add a star.

Algorithm:

1. STEP 1: Declaring a recursive function pairStar with parameters (char arr[], int start)
2. STEP 2: Base Case: if(arr[start] == '\0') return;
3. STEP 3: if(start == start +1)
Shift all the letters from start+1 to right side by 1.
4. STEP 4: Enter '*' at (start +1) in arr.

Example:

```    Input = "aaa"
First Traversal: "aa*a"
Second Traversal: "a*a*a"```

C++ program:

``````#include <iostream>
using namespace std;

//Function To find length
int length(char arr[]){
int len = 0;
for(int i =0;arr[i]!='\0';i++){
len++;
}
return len;
}

//Recursive Function
void pairStar(char arr[],int start){
//Base Case: Reached End Of String
if(arr[start]=='\0'){
return;
}
//Recursive Call
pairStar(arr,start+1);
if(arr[start] == arr[start+1]){
int l = length(arr);
//Extending the string
arr[l+1] = '\0';
int i;
//To shift the letters by 1
for(i = l-1;i>=start +1;i--){
arr[i+1] = arr [i];
}
//Entering * in between
arr[start+1] = '*';
}
}

//Main
int main(){
char input[50];
cout<<"Enter Input"<<endl;
cin>> input;

pairStar(input,0);  //Calling the function

cout<<"Modified Output"<<endl;
cout<<input<<endl;

return 0;
}
``````

Output

```Enter Input
hello
Modified Output
hel*lo
```