# Check if a binary string has two consecutive occurrences of one everywhere

• Difficulty Level : Basic
• Last Updated : 27 May, 2021

Given string str consisting of only the characters ‘a’ and ‘b’, the task is to check whether the string is valid or not. In a valid string, every group of consecutive b must be of length 2 and must appear after 1 or more occurrences of character ‘a’ i.e. “abba” is a valid sub-string but “abbb” and aba are not. Print 1 if the string is valid, else print -1.

Examples:

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.

Input: str = “abbaaabbabba”
Output: 1

Input: str = “abbaaababb”
Output: -1

Approach: Find every occurrence of ‘b’ in the string and check whether it is a part of the sub-string “abb”. If the condition fails for any sub-string, then print -1 else print 1.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;` `// Function that returns 1 if str is valid``bool` `isValidString(string str, ``int` `n)``{``    ``// Index of first appearance of 'b'``    ``int` `index = find(str.begin(),      ``                     ``str.end(), ``'b'``) -``                     ``str.begin();` `    ``// If str starts with 'b'``    ``if` `(index == 0)``        ``return` `false``;` `    ``// While 'b' occurs in str``    ``while` `(index <= n - 1)``    ``{``        ``// If 'b' doesn't appear after an 'a'``        ``if` `(str[index - 1] != ``'a'``)``            ``return` `false``;` `        ``// If 'b' is not succeeded by another 'b'``        ``if` `(index + 1 < n && str[index + 1] != ``'b'``)``            ``return` `false``;` `        ``// If sub-string is of the type "abbb"``        ``if` `(index + 2 < n && str[index + 2] == ``'b'``)``            ``return` `false``;` `        ``// If str ends with a single b``        ``if` `(index == n - 1)``            ``return` `false``;` `        ``index = find(str.begin() + index + 2,``                     ``str.end(), ``'b'``) - str.begin();``    ``}``    ``return` `true``;``}` `// Driver code``int` `main()``{``    ``string str = ``"abbaaabbabba"``;``    ``int` `n = str.length();``    ``isValidString(str, n) ? cout``                ``<< ``"true"` `: cout << ``"false"``;``    ``return` `0;``}` `// This code is contributed by``// sanjeev2552`

## Java

 `// Java implementation of the approach``class` `GFG {` `    ``// Function that returns 1 if str is valid``    ``private` `static` `boolean` `isValidString(String str, ``int` `n)``    ``{` `        ``// Index of first appearance of 'b'``        ``int` `index = str.indexOf(``"b"``);` `        ``// If str starts with 'b'``        ``if` `(index == ``0``)``            ``return` `false``;` `        ``// While 'b' occurs in str``        ``while` `(index != -``1``) {` `            ``// If 'b' doesn't appear after an 'a'``            ``if` `(str.charAt(index - ``1``) != ``'a'``)``                ``return` `false``;` `            ``// If 'b' is not succeeded by another 'b'``            ``if` `(index + ``1` `< n && str.charAt(index + ``1``) != ``'b'``)``                ``return` `false``;` `            ``// If sub-string is of the type "abbb"``            ``if` `(index + ``2` `< n && str.charAt(index + ``2``) == ``'b'``)``                ``return` `false``;` `            ``// If str ends with a single b``            ``if` `(index == n - ``1``)``                ``return` `false``;` `            ``index = str.indexOf(``"b"``, index + ``2``);``        ``}``        ``return` `true``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String str = ``"abbaaabbabba"``;``        ``int` `n = str.length();``        ``System.out.println(isValidString(str, n));``    ``}``}`

## Python 3

 `# Python 3 implementation of the approach` `# Function that returns 1 if str is valid``def` `isValidString(``str``, n):` `    ``# Index of first appearance of 'b'``    ``idx ``=` `str``.find(``"b"``)` `    ``# If str starts with 'b'``    ``if` `(idx ``=``=` `0``):``        ``return` `False` `    ``# While 'b' occurs in str``    ``while` `(idx !``=` `-``1``):` `        ``# If 'b' doesn't appear after an 'a'``        ``if` `(``str``[idx ``-` `1``] !``=` `'a'``):``            ``return` `False` `        ``# If 'b' is not succeeded by another 'b'``        ``if` `(idx ``+` `1` `< n ``and` `str``[idx ``+` `1``] !``=` `'b'``):``            ``return` `False` `        ``# If sub-string is of the type "abbb"``        ``if` `(idx ``+` `2` `< n ``and` `str``[idx ``+` `2``] ``=``=` `'b'``):``            ``return` `False` `        ``# If str ends with a single b``        ``if` `(idx ``=``=` `n ``-` `1``):``            ``return` `False` `        ``idx ``=` `str``.find(``"b"``, idx ``+` `2``)` `    ``return` `True` `# Driver code``if` `__name__ ``=``=` `"__main__"``:` `    ``str` `=` `"abbaaabbabba"``    ``n ``=` `len``(``str``)``    ``print``(isValidString(``str``, n))` `# This code is contributed by ita_c`

## C#

 `// C# implementation of the approach``using` `System;` `class` `GFG``{` `    ``// Function that returns 1 if str is valid``    ``private` `static` `bool` `isValidString(``string` `str, ``int` `n)``    ``{` `        ``// Index of first appearance of 'b'``        ``int` `index = str.IndexOf(``"b"``);` `        ``// If str starts with 'b'``        ``if` `(index == 0)``            ``return` `false``;` `        ``// While 'b' occurs in str``        ``while` `(index != -1)``        ``{` `            ``// If 'b' doesn't appear after an 'a'``            ``if` `(str[index - 1] != ``'a'``)``                ``return` `false``;` `            ``// If 'b' is not succeeded by another 'b'``            ``if` `(index + 1 < n && str[index + 1] != ``'b'``)``                ``return` `false``;` `            ``// If sub-string is of the type "abbb"``            ``if` `(index + 2 < n && str[index + 2] == ``'b'``)``                ``return` `false``;` `            ``// If str ends with a single b``            ``if` `(index == n - 1)``                ``return` `false``;` `            ``index = str.IndexOf(``"b"``, index + 2);``        ``}``        ``return` `true``;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main()``    ``{``        ``string` `str = ``"abbaaabbabba"``;``        ``int` `n = str.Length;``        ``Console.WriteLine(isValidString(str, n));``    ``}``}` `// This code is contributed by Ryuga`

## Javascript

 ``
Output:
`true`

My Personal Notes arrow_drop_up