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Count number of binary strings such that there is no substring of length greater than or equal to 3 with all 1’s
  • Difficulty Level : Medium
  • Last Updated : 14 Apr, 2021

Given an integer N, the task is to count the number of binary strings possible of length N such that they don’t contain “111” as a substring. The answer could be large so print answer modulo 109 + 7.
Examples: 
 

Input: N = 3 
Output:
All possible substring are “000”, “001”, 
“010”, “011”, “100”, “101” and “110”. 
“111” is not a valid string.
Input N = 16 
Output: 19513 
 

 

Approach: Dynamic programming can be used to solve this problem. Create a dp[][] array where dp[i][j] will store the count of possible substrings such that 1 appears j times consecutively upto the ith index. Now, the recurrence relations will be: 
 

dp[i][0] = dp[i – 1][0] + dp[i – 1][1] + dp[i – 1][2] 
dp[i][1] = dp[i – 1][0] 
dp[i][2] = dp[i – 1][1] 
 



And the base cases will be dp[1][0] = 1, dp[1][1] = 1 and dp[1][2] = 0. Now, the required count of strings will be dp[N][0] + dp[N][1] + dp[N][2].
Below is the implementation of the above approach: 
 

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
const long MOD = 1000000007;
 
// Function to return the count
// of all possible valid strings
long countStrings(long N)
{
 
    long dp[N + 1][3];
 
    // Fill 0's in the dp array
    memset(dp, 0, sizeof(dp));
 
    // Base cases
    dp[1][0] = 1;
    dp[1][1] = 1;
    dp[1][2] = 0;
 
    for (int i = 2; i <= N; i++) {
 
        // dp[i][j] = number of possible strings
        // such that '1' just appeared consecutively
        // j times upto the ith index
        dp[i][0] = (dp[i - 1][0] + dp[i - 1][1]
                    + dp[i - 1][2])
                   % MOD;
 
        // Taking previously calculated value
        dp[i][1] = dp[i - 1][0] % MOD;
        dp[i][2] = dp[i - 1][1] % MOD;
    }
 
    // Taking all possible cases that
    // can appear at the Nth position
    long ans = (dp[N][0] + dp[N][1]
                + dp[N][2])
               % MOD;
 
    return ans;
}
 
// Driver code
int main()
{
    long N = 3;
 
    cout << countStrings(N);
 
    return 0;
}

Java




// Java implementation of the approach
class GFG
{
    final static int MOD = 1000000007;
     
    // Function to return the count
    // of all possible valid strings
    static long countStrings(int N)
    {
        int i, j;
         
        int dp[][] = new int[N + 1][3];
     
        // Fill 0's in the dp array
        for(i = 0; i < N + 1; i++)
        {
            for(j = 9; j < 3 ; j ++)
            {
                dp[i][j] = 0;
            }
        }
         
        // Base cases
        dp[1][0] = 1;
        dp[1][1] = 1;
        dp[1][2] = 0;
     
        for (i = 2; i <= N; i++)
        {
     
            // dp[i][j] = number of possible strings
            // such that '1' just appeared consecutively
            // j times upto the ith index
            dp[i][0] = (dp[i - 1][0] + dp[i - 1][1] +
                        dp[i - 1][2]) % MOD;
     
            // Taking previously calculated value
            dp[i][1] = dp[i - 1][0] % MOD;
            dp[i][2] = dp[i - 1][1] % MOD;
        }
     
        // Taking all possible cases that
        // can appear at the Nth position
        int ans = (dp[N][0] + dp[N][1] +
                              dp[N][2]) % MOD;
     
        return ans;
    }
     
    // Driver code
    public static void main (String[] args)
    {
        int N = 3;
     
        System.out.println(countStrings(N));
    }
}
 
// This code is contributed by AnkitRai01

Python3




# Python3 implementation of the approach
MOD = 1000000007
 
# Function to return the count
# of all possible valid strings
def countStrings(N):
 
    # Initialise and fill 0's in the dp array
    dp = [[0] * 3 for i in range(N + 1)]
 
    # Base cases
    dp[1][0] = 1;
    dp[1][1] = 1;
    dp[1][2] = 0;
 
    for i in range(2, N + 1):
 
        # dp[i][j] = number of possible strings
        # such that '1' just appeared consecutively
        # j times upto the ith index
        dp[i][0] = (dp[i - 1][0] +
                    dp[i - 1][1] +
                    dp[i - 1][2]) % MOD
 
        # Taking previously calculated value
        dp[i][1] = dp[i - 1][0] % MOD
        dp[i][2] = dp[i - 1][1] % MOD
     
 
    # Taking all possible cases that
    # can appear at the Nth position
    ans = (dp[N][0] + dp[N][1] + dp[N][2]) % MOD
 
    return ans
 
# Driver code
if __name__ == '__main__':
 
    N = 3
 
    print(countStrings(N))
 
# This code is contributed by ashutosh450

C#




// C# implementation of the above approach
using System;        
 
class GFG
{
    static readonly int MOD = 1000000007;
     
    // Function to return the count
    // of all possible valid strings
    static long countStrings(int N)
    {
        int i, j;
         
        int [,]dp = new int[N + 1, 3];
     
        // Fill 0's in the dp array
        for(i = 0; i < N + 1; i++)
        {
            for(j = 9; j < 3; j ++)
            {
                dp[i, j] = 0;
            }
        }
         
        // Base cases
        dp[1, 0] = 1;
        dp[1, 1] = 1;
        dp[1, 2] = 0;
     
        for (i = 2; i <= N; i++)
        {
     
            // dp[i,j] = number of possible strings
            // such that '1' just appeared consecutively
            // j times upto the ith index
            dp[i, 0] = (dp[i - 1, 0] + dp[i - 1, 1] +
                        dp[i - 1, 2]) % MOD;
     
            // Taking previously calculated value
            dp[i, 1] = dp[i - 1, 0] % MOD;
            dp[i, 2] = dp[i - 1, 1] % MOD;
        }
     
        // Taking all possible cases that
        // can appear at the Nth position
        int ans = (dp[N, 0] + dp[N, 1] +
                              dp[N, 2]) % MOD;
     
        return ans;
    }
     
    // Driver code
    public static void Main (String[] args)
    {
        int N = 3;
     
        Console.WriteLine(countStrings(N));
    }
}
 
// This code is contributed by Rajput-Ji

Javascript




<script>
 
// javascript implementation of the approach
 
    var MOD = 1000000007;
     
    // Function to return the count
    // of all possible valid strings
    function countStrings(N)
    {
        var i, j;
         
        var dp = Array(N+1).fill(0).map(x => Array(3).fill(0));
     
        // Fill 0's in the dp array
        for(i = 0; i < N + 1; i++)
        {
            for(j = 9; j < 3 ; j ++)
            {
                dp[i][j] = 0;
            }
        }
         
        // Base cases
        dp[1][0] = 1;
        dp[1][1] = 1;
        dp[1][2] = 0;
     
        for (i = 2; i <= N; i++)
        {
     
            // dp[i][j] = number of possible strings
            // such that '1' just appeared consecutively
            // j times upto the ith index
            dp[i][0] = (dp[i - 1][0] + dp[i - 1][1] +
                        dp[i - 1][2]) % MOD;
     
            // Taking previously calculated value
            dp[i][1] = dp[i - 1][0] % MOD;
            dp[i][2] = dp[i - 1][1] % MOD;
        }
     
        // Taking all possible cases that
        // can appear at the Nth position
        var ans = (dp[N][0] + dp[N][1] +
                              dp[N][2]) % MOD;
     
        return ans;
    }
     
// Driver code
var N = 3;
document.write(countStrings(N));
 
// This code is contributed by 29AjayKumar
</script>
Output: 
7

 

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