# Largest sub-string of a binary string divisible by 2

Given a binary string str of length N, the task is to find the longest sub-string divisible by 2. If no such sub-string exists then print -1.

Examples:

Input: str = “11100011”
Output: 111000
Largest sub-string divisible by 2 is “111000”.

Input: str = “1111”
Output: -1
There is no sub-string of the given string
which is divisible by 2.

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

Naive approach: A naive approach will be to generate all such sub-strings and check if they are divisible by 2. The time complexity of this approach will be O(N3).

Better approach: A straight forward approach will be to remove characters from the end of the string while the last character is 1. The moment a 0 is encountered, the current string will be divisible by 2 as it ends at a 0. The time complexity of this approach will be O(N).

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach ` `#include ` `using` `namespace` `std; ` ` `  `// Function to return the largest ` `// substring divisible by 2 ` `string largestSubStr(string s) ` `{ ` `    ``// While the last character of ` `    ``// the string is '1', pop it ` `    ``while` `(s.size() and s[s.size() - 1] == ``'1'``) ` `        ``s.pop_back(); ` ` `  `    ``// If the original string had no '0' ` `    ``if` `(s.size() == 0) ` `        ``return` `"-1"``; ` `    ``else` `        ``return` `s; ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``string s = ``"11001"``; ` ` `  `    ``cout << largestSubStr(s); ` ` `  `    ``return` `0; ` `} `

## Java

 `// Java implementation of the approach  ` `class` `GFG ` `{ ` `     `  `    ``// Function to return the largest  ` `    ``// substring divisible by 2  ` `    ``static` `String largestSubStr(String s)  ` `    ``{  ` `        ``// While the last character of  ` `        ``// the string is '1', pop it  ` `        ``while` `(s.length() != ``0` `&&  ` `               ``s.charAt(s.length() - ``1``) == ``'1'``)  ` `            ``s = s.substring(``0``, s.length() - ``1``);  ` `     `  `        ``// If the original string had no '0'  ` `        ``if` `(s.length() == ``0``)  ` `            ``return` `"-1"``;  ` `        ``else` `            ``return` `s;  ` `    ``}  ` `     `  `    ``// Driver code  ` `    ``public` `static` `void` `main (String[] args) ` `    ``{  ` `        ``String s = ``"11001"``;  ` `     `  `        ``System.out.println(largestSubStr(s));  ` `    ``}  ` `} ` ` `  `// This code is contributed by AnkitRai01 `

## Python3

 `# Python3 implementation of the approach  ` ` `  `# Function to return the largest  ` `# substring divisible by 2  ` `def` `largestSubStr(s) :  ` ` `  `    ``# While the last character of  ` `    ``# the string is '1', pop it  ` `    ``while` `(``len``(s) ``and` `s[``len``(s) ``-` `1``] ``=``=` `'1'``) : ` `        ``s ``=` `s[:``len``(s) ``-` `1``];  ` ` `  `    ``# If the original string had no '0'  ` `    ``if` `(``len``(s) ``=``=` `0``) : ` `        ``return` `"-1"``;  ` `    ``else` `: ` `        ``return` `s;  ` ` `  `# Driver code  ` `if` `__name__ ``=``=` `"__main__"` `: ` ` `  `    ``s ``=` `"11001"``;  ` ` `  `    ``print``(largestSubStr(s));  ` ` `  `# This code is contributed by AnkitRai01 `

## C#

 `// C# implementation of the approach  ` `using` `System; ` ` `  `class` `GFG  ` `{  ` `     `  `    ``// Function to return the largest  ` `    ``// substring divisible by 2  ` `    ``static` `string` `largestSubStr(``string` `s)  ` `    ``{  ` `        ``// While the last character of  ` `        ``// the string is '1', pop it  ` `        ``while` `(s.Length != 0 &&  ` `               ``s[s.Length - 1] == ``'1'``)  ` `            ``s = s.Substring(0, s.Length - 1);  ` `     `  `        ``// If the original string had no '0'  ` `        ``if` `(s.Length == 0)  ` `            ``return` `"-1"``;  ` `        ``else` `            ``return` `s;  ` `    ``}  ` `     `  `    ``// Driver code  ` `    ``public` `static` `void` `Main ()  ` `    ``{  ` `        ``string` `s = ``"11001"``;  ` `     `  `        ``Console.WriteLine(largestSubStr(s));  ` `    ``}  ` `}  ` ` `  `// This code is contributed by AnkitRai01  `

Output:

```1100
```

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