Check if a string follows a^nb^n pattern or not

Given a string str, return true string follows pattern anbn, i.e., it has a’s followed by b’s such that the number of a’s and b’s are same.

Examples:

Input : str = "aabb"
Output : Yes

Input : str = "abab"
Output : No

Input : str = "aabbb"
Output : No



The idea is to first count a’s. If number of a’s is not equal to half of string’s length, then return false. Else check if all remaining characters are b’s or not.

Below is the implementation of above idea :

C/C++

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// C++ program to check if a string is of
// the form a^nb^n.
#include<iostream>
using namespace std;
  
// Returns true str is of the form a^nb^n.
bool isAnBn(string str)
{
   int n = str.length();
  
   // After this loop 'i' has count of a's
   int i;
   for (i=0; i<n; i++)
      if (str[i] != 'a')
         break;
  
   // Since counts of a's and b's should
   // be equal, a should apear exactly
   // n/2 times
   if (i*2 != n)
      return false;
  
   // Rest of the characters must be all 'b'
   int j;
   for (j=i; j<n; j++)
      if (str[j] != 'b')
         return false;
  
   return true;
}
  
// Driver code
int main()
{
    string str = "abab";
    isAnBn(str) ? cout << "Yes"
                : cout << "No";
    return 0;
}

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Java

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// Java program to check if a string is of
// the form a^nb^n.
import java.util.*;
import java.lang.*;
import java.io.*;
  
class CheckPattern
{
    public static boolean isAnBn(String s)
    {
        int l = s.length();
          
        // Only even length strings will have same number of a's and b's
        if (l%2 == 1)
        {
            return false;
        }
        // Set two pointers, one from the left and another from right
        int i = 0;
        int j = l-1;
          
        // Compare the characters till the center
        while (i<j)
        {
            if(s.charAt(i) != 'a' || s.charAt(j) != 'b')
            {
                return false;
            }
            i++;
            j--;
        }
        return true;    
    }
      
    public static void main (String[] args) throws java.lang.Exception
    {
        String s = "abab";
          
        // Function call
        boolean value = isAnBn(s);
        if(value == true){
            System.out.println("Yes");
        }
        else{
            System.out.println("No");
        }
    }
}
  
// Code contributed by Shivani Sanjay Shinde.

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Python 3

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# Python 3program to check if a 
# string is of the form a^nb^n.
  
# Returns true str is of the 
# form a^nb^n.
def isAnBn(str):
      
    n = len(str)
      
    # After this loop 'i' has 
    # count of a's
    for i in range(n):
        if (str[i] != 'a'):
            break
      
    # Since counts of a's and b's should
    # be equal, a should apear exactly
    # n/2 times
    if (i * 2 != n):
        return False
      
    # Rest of the characters must
    # be all 'b'
    for j in range(i, n):
        if (str[j] != 'b'):
            return False
      
    return True
  
# Driver code
if __name__ == "__main__":
    str = "abab"
    print("Yes") if isAnBn(str) else print("No")
  
# This code is contributed 
# by ChitraNayal

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C#

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// C# program to check if a string 
// is of the form a^ nb ^ n.
using System;
  
class GFG {
      
    // Function returns true str is of the form a^nb^n.
    public static bool isAnBn(String s)
    {
        int l = s.Length;
          
        // Only even length strings will have
        // same number of a's and b's
        if (l % 2 == 1)
        {
            return false;
        }
          
        // Set two pointers, one from the
        // left and another from right
        int i = 0;
        int j = l - 1;
          
        // Compare the characters 
        // till the center
        while (i < j)
        {
            if(s[i] != 'a' || s[j] != 'b')
            {
                return false;
            }
            i++;
            j--;
        }
        return true
    }
      
    // Driver Code
    public static void Main () 
    {
        String s = "abab";
          
        // Function call
        bool value = isAnBn(s);
        if(value == true)
        {
            Console.Write("Yes");
        }
        else
        {
            Console.Write("No");
        }
    }
}
  
// This code is contributed by Nitin Mittal.

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PHP

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<?php
// PHP program to check if a string 
// is of the form a^nb^n.
  
// Returns true str is of
// the form a^nb^n.
function isAnBn($str)
{
    $n = strlen($str);
      
    // After this loop 'i' 
    // has count of a's
    $i;
    for($i = 0; $i < $n; $i++)
        if ($str[$i] != 'a')
            break;
      
    // Since counts of a's and b's should
    // be equal, a should apear exactly
    // n/2 times
    if ($i * 2 != $n)
        return false;
      
    // Rest of the characters
    // must be all 'b'
    $j;
    for($j = $i; $j < $n; $j++)
        if ($str[$j] != 'b')
            return false;
      
    return true;
}
  
    // Driver code
    $str = "abab";
    if(isAnBn($str)) 
        echo "Yes";
    else    
        echo "No";
  
// This code is contributed by nitin mittal.
?>

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Output:

No

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Improved By : nitin mittal, chitranayal



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