Check if a given string is made up of two alternating characters

Given a string str, the task is to check whether the given string is made up of only two alternating characters.
Examples:

Input: str = “ABABABAB” 
Output: Yes
Input: str = “XYZ” 
Output: No 

Approach: In order for the string to be made up of only two alternating characters, it must satisfy the following conditions: 

1. All the characters at odd indices must be same.
2. All the characters at even indices must be same.
3. str[0] != str[1] (This is because string of type “AAAAA” where a single character is repeated a number of time will also satisfy the above two conditions)

Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function that returns true if the string
// is made up of two alternating characters
bool isTwoAlter(string s)
{

    // Check if ith character matches
    // with the character at index (i + 2)
    for (int i = 0; i < s.length() - 2; i++) {
        if (s[i] != s[i + 2]) {
            return false;
        }
    }

    // If string consists of a single
    // character repeating itself
    if (s[0] == s[1])
        return false;

    return true;
}

// Driver code
int main()
{
    string str = "ABAB";

    if (isTwoAlter(str))
        cout << "Yes";
    else
        cout << "No";

    return 0;
}

Java

// Java implementation of the approach
import java.io.*;

class GFG
{

// Function that returns true if the string
// is made up of two alternating characters
static boolean isTwoAlter(String s)
{

    // Check if ith character matches
    // with the character at index (i + 2)
    for (int i = 0; i < s.length() - 2; i++)
    {
        if (s.charAt(i) != s.charAt(i + 2))
        {
            return false;
        }
    }

    // If string consists of a single
    // character repeating itself
    if (s.charAt(0) == s.charAt(1))
        return false;

    return true;
}

// Driver code
public static void main (String[] args)
{
        String str = "ABAB";

    if (isTwoAlter(str))
        System.out.print( "Yes");
    else
        System.out.print("No");
}
}

// This code is contributed by anuj_67..

Python 3

# Function that returns true if the string
# is made up of two alternating characters
def isTwoAlter( s):

    # Check if ith character matches
    # with the character at index (i + 2)
    for i in range ( len( s) - 2) :
        if (s[i] != s[i + 2]) :
            return False



    #If string consists of a single
    #character repeating itself
    if (s[0] == s[1]):
        return False

    return True

# Driver code
if __name__ == "__main__":
    str = "ABAB"

    if (isTwoAlter(str)):
        print ( "Yes")
    else:
        print ("No")

# This code is contributed by ChitraNayal

C#

// C# implementation of the approach
using System;

class GFG
{

    // Function that returns true if the string
    // is made up of two alternating characters
    static bool isTwoAlter(string s)
    {

        // Check if ith character matches
        // with the character at index (i + 2)
        for (int i = 0; i < s.Length - 2; i++)
        {
            if (s[i] != s[i +2])
            {
                return false;
            }
        }

        // If string consists of a single
        // character repeating itself
        if (s[0] == s[1])
            return false;

        return true;
    }

    // Driver code
    public static void Main()
    {
            string str = "ABAB";

        if (isTwoAlter(str))
            Console.WriteLine( "Yes");
        else
            Console.WriteLine("No");
    }
}

// This code is contributed by AnkitRai01

Javascript

<script>

// Javascript implementation of the approach

    // Function that returns true if the string
    // is made up of two alternating characters
    function isTwoAlter(s)
    {
        // Check if ith character matches
    // with the character at index (i + 2)
    for (let i = 0; i < s.length - 2; i++)
    {
        if (s[i] != s[i+2])
        {
            return false;
        }
    }

    // If string consists of a single
    // character repeating itself
    if (s[0] == s[1])
        return false;

    return true;
    }

    // Driver code
    let str = "ABAB";
    if (isTwoAlter(str))
        document.write( "Yes");
    else
        document.write("No");

    // This code is contributed by rag2127

</script>

Output:

Yes

Time Complexity : O(N) 
Auxiliary Space : O(1)