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Count strings with consecutive 1’s

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  • Difficulty Level : Medium
  • Last Updated : 18 Jan, 2022
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Given a number n, count number of n length strings with consecutive 1’s in them.

Examples: 

Input  : n = 2
Output : 1
There are 4 strings of length 2, the
strings are 00, 01, 10 and 11. Only the 
string 11 has consecutive 1's.

Input  : n = 3
Output : 3
There are 8 strings of length 3, the
strings are 000, 001, 010, 011, 100, 
101, 110 and 111.  The strings with
consecutive 1's are 011, 110 and 111.

Input : n = 5
Output : 19
Recommended Practice

The reverse problem of counting strings without consecutive 1’s can be solved using Dynamic Programming (See the solution here). We can use that solution and find the required count using below steps.

  1. Compute the number of binary strings without consecutive 1’s using the approach discussed here. Let this count be c.
  2. Count of all possible binary strings of length n is 2^n.
  3. Total binary strings with consecutive 1 is 2^n – c.

Below is the implementation of the above steps. 

C++




// C++ program to count all distinct
// binary strings with two consecutive 1's
#include <iostream>
using namespace std;
 
// Returns count of n length binary
// strings with consecutive 1's
int countStrings(int n)
{
    // Count binary strings without consecutive 1's.
    // See the approach discussed on be
    // ( http://goo.gl/p8A3sW )
    int a[n], b[n];
    a[0] = b[0] = 1;
    for (int i = 1; i < n; i++)
    {
        a[i] = a[i - 1] + b[i - 1];
        b[i] = a[i - 1];
    }
 
    // Subtract a[n-1]+b[n-1] from 2^n
    return (1 << n) - a[n - 1] - b[n - 1];
}
 
// Driver code
int main()
{
    cout << countStrings(5) << endl;
    return 0;
}

Java




// Java program to count all distinct
// binary strings with two consecutive 1's
 
class GFG {
 
    // Returns count of n length binary
    // strings with consecutive 1's
    static int countStrings(int n)
    {
        // Count binary strings without consecutive 1's.
        // See the approach discussed on be
        // ( http://goo.gl/p8A3sW )
        int a[] = new int[n], b[] = new int[n];
        a[0] = b[0] = 1;
 
        for (int i = 1; i < n; i++) {
            a[i] = a[i - 1] + b[i - 1];
            b[i] = a[i - 1];
        }
 
        // Subtract a[n-1]+b[n-1]
        from 2 ^ n return (1 << n) - a[n - 1] - b[n - 1];
    }
 
    // Driver code
    public static void main(String args[])
    {
        System.out.println(countStrings(5));
    }
}
 
// This code is contributed by Nikita tiwari.

Python 3




# Python 3 program to count all
# distinct binary strings with
# two consecutive 1's
 
 
# Returns count of n length
# binary strings with
# consecutive 1's
def countStrings(n):
 
    # Count binary strings without
    # consecutive 1's.
    # See the approach discussed on be
    # ( http://goo.gl/p8A3sW )
    a = [0] * n
    b = [0] * n
    a[0] = b[0] = 1
    for i in range(1, n):
        a[i] = a[i - 1] + b[i - 1]
        b[i] = a[i - 1]
 
    # Subtract a[n-1]+b[n-1] from 2^n
    return (1 << n) - a[n - 1] - b[n - 1]
 
 
# Driver code
print(countStrings(5))
 
 
# This code is contributed
# by Nikita tiwari.

C#




// program to count all distinct
// binary strings with two
// consecutive 1's
using System;
 
class GFG {
 
    // Returns count of n length
    // binary strings with
    // consecutive 1's
    static int countStrings(int n)
    {
        // Count binary strings without
        // consecutive 1's.
        // See the approach discussed on
        // ( http://goo.gl/p8A3sW )
        int[] a = new int[n];
        int[] b = new int[n];
        a[0] = b[0] = 1;
 
        for (int i = 1; i < n; i++)
        {
            a[i] = a[i - 1] + b[i - 1];
            b[i] = a[i - 1];
        }
 
        // Subtract a[n-1]+b[n-1]
        // from 2^n
        return (1 << n) - a[n - 1] - b[n - 1];
    }
 
    // Driver code
    public static void Main()
    {
        Console.WriteLine(countStrings(5));
    }
}
 
// This code is contributed by Anant Agarwal.

PHP




<?php
// PHP program to count all
// distinct binary strings
// with two consecutive 1's
// Returns count of n length binary
// strings with consecutive 1's
 
function countStrings($n)
{
     
    // Count binary strings without consecutive 1's.
    // See the approach discussed on be
    // ( http://goo.gl/p8A3sW )
    $a[$n] = 0;
    $b[$n] = 0;
    $a[0] = $b[0] = 1;
    for ($i = 1; $i < $n; $i++)
    {
        $a[$i] = $a[$i - 1] + $b[$i - 1];
        $b[$i] = $a[$i - 1];
    }
 
    // Subtract a[n-1]+b[n-1] from 2^n
    return (1 << $n) - $a[$n - 1] -
                       $b[$n - 1];
}
 
    // Driver Code
    echo countStrings(5), "\n";
 
// This Code is contributed by Ajit
?>

Javascript




<script>
 
// JavaScript program to count all distinct
// binary strings with two
// consecutive 1's
 
    // Returns count of n length binary
    // strings with consecutive 1's
    function countStrings(n)
    {
        // Count binary strings without consecutive 1's.
        // See the approach discussed on be
        // ( http://goo.gl/p8A3sW )
        let a = [], b = [];
        a[0] = b[0] = 1;
  
        for (let i = 1; i < n; i++) {
            a[i] = a[i - 1] + b[i - 1];
            b[i] = a[i - 1];
        }
  
        // Subtract a[n-1]+b[n-1]
        // from 2 ^ n
        return (1 << n) - a[n - 1] - b[n - 1];
    }
 
// Driver Code
 
       document.write(countStrings(5));
 
</script>

Output

19

Time Complexity: O(n)

Auxiliary Space: O(n)

Optimization: 
The time complexity of the above solution is O(n). We can optimize the above solution to work in O(Logn). 
If we take a closer look at the pattern of counting strings without consecutive 1’s, we can observe that the count is actually (n+2)th Fibonacci number for n >= 1. The Fibonacci Numbers are 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 141, ….

n = 1, count = 0  = 21 - fib(3) 
n = 2, count = 1  = 22 - fib(4)
n = 3, count = 3  = 23 - fib(5)
n = 4, count = 8  = 24 - fib(6)
n = 5, count = 19 = 25 - fib(7)
................

We can find n’th Fibonacci Number in O(Log n) time (See method 4 here). 
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
 


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