Find the number of binary strings of length N with at least 3 consecutive 1s

Given an integer N. The task is to find the number of all possible distinct binary strings of length N which have at least 3 consecutive 1s.
Examples:

Input: N = 3 
Output: 1 
The only string of length 3 possible is “111”.
Input: N = 4 
Output: 3 
The 3 strings are “1110”, “0111” and “1111”. 

Naive approach: Consider all possible strings.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Function that returns true if s contains
// three consecutive 1's
bool check(string& s)
{
    int n = s.length();
    for (int i = 2; i < n; i++) {
        if (s[i] == '1' && s[i - 1] == '1' && s[i - 2] == '1')
            return 1;
    }
    return 0;
}

// Function to return the count
// of required strings
int countStr(int i, string& s)
{
    if (i < 0) {
        if (check(s))
            return 1;
        return 0;
    }
    s[i] = '0';
    int ans = countStr(i - 1, s);
    s[i] = '1';
    ans += countStr(i - 1, s);
    s[i] = '0';
    return ans;
}

// Driver code
int main()
{
    int N = 4;
    string s(N, '0');
    cout << countStr(N - 1, s);

    return 0;
}

Java

// Java implementation of the approach
import java.util.*;

class GFG
{

// Function that returns true if s contains
// three consecutive 1's
static boolean check(String s)
{
    int n = s.length();
    for (int i = 2; i < n; i++)
    {
        if (s.charAt(i) == '1' &&
          s.charAt(i-1) == '1' &&
          s.charAt(i-2) == '1')
            return true;
    }
    return false;
}

// Function to return the count
// of required strings
static int countStr(int i, String s)
{
    if (i < 0)
    {
        if (check(s))
            return 1;
        return 0;
    }
    char[] myNameChars = s.toCharArray();
    myNameChars[i] = '0';
    s = String.valueOf(myNameChars);

    int ans = countStr(i - 1, s);
    char[] myChar = s.toCharArray();
    myChar[i] = '1';
    s = String.valueOf(myChar);

    ans += countStr(i - 1, s);
    char[]myChar1 = s.toCharArray();
    myChar1[i] = '0';
    s = String.valueOf(myChar1);

    return ans;
}

// Driver code
public static void main(String args[])
{
    int N = 4;
    String s = "0000";
    System.out.println(countStr(N - 1, s));
}
}

// This code is contributed by
// Surendra_Gangwar

Python3

# Python3 implementation of the approach

# Function that returns true if s contains
# three consecutive 1's
def check(s) :
    n = len(s);
    for i in range(2, n) :
        if (s[i] == '1' and s[i - 1] == '1' and
                            s[i - 2] == '1') :
            return 1;

# Function to return the count
# of required strings
def countStr(i, s) :

    if (i < 0) :
        if (check(s)) :
            return 1;
        return 0;

    s[i] = '0';
    ans = countStr(i - 1, s);

    s[i] = '1';
    ans += countStr(i - 1, s);

    s[i] = '0';
    return ans;

# Driver code
if __name__ == "__main__" :
    N = 4;
    s = list('0' * N);

    print(countStr(N - 1, s));

# This code is contributed by Ryuga

C#

// C# implementation of the approach
// value x
using System;

class GFG
{

// Function that returns true if s contains
// three consecutive 1's
static bool check(String s)
{
    int n = s.Length;
    for (int i = 2; i < n; i++)
    {
        if (s[i] == '1' && s[i - 1] == '1' && s[i - 2] == '1')
            return true;
    }
    return false;
}

// Function to return the count
// of required strings
static int countStr(int i, String s)
{
    if (i < 0)
    {
        if (check(s))
            return 1;
        return 0;
    }
    char[] myNameChars = s.ToCharArray();
    myNameChars[i] = '0';
    s = String.Join("", myNameChars);

    int ans = countStr(i - 1, s);
    char[] myChar = s.ToCharArray();
    myChar[i] = '1';
    s = String.Join("", myChar);

    ans += countStr(i - 1, s);
    char[]myChar1 = s.ToCharArray();
    myChar1[i] = '0';
    s = String.Join("", myChar1);

    return ans;
}

// Driver code
public static void Main(String []args)
{
    int N = 4;
    String s = "0000";
    Console.WriteLine(countStr(N - 1, s));
}
}

/* This code contributed by PrinciRaj1992 */

Javascript

<script>

// JavaScript implementation of the approach
// value x

// Function that returns true if s contains
// three consecutive 1's
function check(s)
{
    let n = s.length;
    for (let i = 2; i < n; i++)
    {
        if (s[i] == '1' &&
          s[i-1] == '1' &&
          s[i-2] == '1')
            return true;
    }
    return false;
}

// Function to return the count
// of required strings
function countStr(i,s)
{
    if (i < 0)
    {
        if (check(s))
            return 1;
        return 0;
    }
    let myNameChars = s.split("");
    myNameChars[i] = '0';
    s = (myNameChars).join("");

    let ans = countStr(i - 1, s);
    let myChar = s.split("");
    myChar[i] = '1';
    s = (myChar).join("");

    ans += countStr(i - 1, s);
    let myChar1 = s.split("");
    myChar1[i] = '0';
    s = (myChar1).join("");

    return ans;
}

// Driver code
let N = 4;
let s = "0000";
document.write(countStr(N - 1, s));


// This code is contributed by avanitrachhadiya2155

</script>

Output:

3

Time Complexity: O(2^N) 
Space Complexity: O(N) because of recurrence stack space.
Efficient approach: We use dynamic programming for computing the number of strings. 
State of dp: dp(i, x) : denotes number of strings of length i with x consecutive 1s in position i + 1 to i + x. 
Recurrence: dp(i, x) = dp(i – 1, 0) + dp(i – 1, x + 1) 
The recurrence is based on the fact that either the string can have a ‘0’ at position i or a ‘1’. 

1. If it has a ‘0’ at position i then for (i-1)th position value of x = 0.
2. If it has a ‘1’ at position i the for (i-1)th position value of x = value of x at position i + 1.

Base Condition: dp(i, 3) = 2ⁱ. Because once you have 3 consecutive ‘1’s you don’t care what characters are there at position 1, 2…i of string as all the strings are valid.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

int n;

// Function to return the count
// of required strings
int solve(int i, int x, int dp[][4])
{
    if (i < 0)
        return x == 3;
    if (dp[i][x] != -1)
        return dp[i][x];

    // '0' at ith position
    dp[i][x] = solve(i - 1, 0, dp);

    // '1' at ith position
    dp[i][x] += solve(i - 1, x + 1, dp);
    return dp[i][x];
}

// Driver code
int main()
{
    n = 4;
    int dp[n][4];

    for (int i = 0; i < n; i++)
        for (int j = 0; j < 4; j++)
            dp[i][j] = -1;

    for (int i = 0; i < n; i++) {

        // Base condition:
        // 2^(i+1) because of 0 indexing
        dp[i][3] = (1 << (i + 1));
    }
    cout << solve(n - 1, 0, dp);

    return 0;
}

Java

// Java implementation of the above approach
import java.util.*;

class GFG
{

    static int n;

    // Function to return the count
    // of required strings
    static int solve(int i, int x, int dp[][])
    {
        if (i < 0)
        {
            return x == 3 ? 1 : 0;
        }
        if (dp[i][x] != -1)
        {
            return dp[i][x];
        }

        // '0' at ith position
        dp[i][x] = solve(i - 1, 0, dp);

        // '1' at ith position
        dp[i][x] += solve(i - 1, x + 1, dp);
        return dp[i][x];
    }

    // Driver code
    public static void main(String[] args)
    {
        n = 4;
        int dp[][] = new int[n][4];

        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < 4; j++)
            {
                dp[i][j] = -1;
            }
        }

        for (int i = 0; i < n; i++)
        {

            // Base condition:
            // 2^(i+1) because of 0 indexing
            dp[i][3] = (1 << (i + 1));
        }
        System.out.print(solve(n - 1, 0, dp));
    }
}

// This code has been contributed by 29AjayKumar

Python 3

# Python 3 implementation of the approach

# Function to return the count
# of required strings
def solve(i, x, dp):
    if (i < 0):
        return x == 3
    if (dp[i][x] != -1):
        return dp[i][x]

    # '0' at ith position
    dp[i][x] = solve(i - 1, 0, dp)

    # '1' at ith position
    dp[i][x] += solve(i - 1, x + 1, dp)
    return dp[i][x]


# Driver code
if __name__ == "__main__":

    n = 4;
    dp = [[0 for i in range(n)] for j in range(4)]

    for i in range(n):
        for j in range(4):
            dp[i][j] = -1

    for i in range(n) :

        # Base condition:
        # 2^(i+1) because of 0 indexing
        dp[i][3] = (1 << (i + 1))

    print(solve(n - 1, 0, dp))

# This code is contributed by ChitraNayal

C#

// C# implementation of the above approach
using System;

class GFG
{

    static int n;

    // Function to return the count
    // of required strings
    static int solve(int i, int x, int [,]dp)
    {
        if (i < 0)
        {
            return x == 3 ? 1 : 0;
        }
        if (dp[i,x] != -1)
        {
            return dp[i,x];
        }

        // '0' at ith position
        dp[i,x] = solve(i - 1, 0, dp);

        // '1' at ith position
        dp[i,x] += solve(i - 1, x + 1, dp);
        return dp[i,x];
    }

    // Driver code
    public static void Main(String[] args)
    {
        n = 4;
        int [,]dp = new int[n, 4];

        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < 4; j++)
            {
                dp[i, j] = -1;
            }
        }

        for (int i = 0; i < n; i++)
        {

            // Base condition:
            // 2^(i+1) because of 0 indexing
            dp[i,3] = (1 << (i + 1));
        }
        Console.Write(solve(n - 1, 0, dp));
    }
}

// This code contributed by Rajput-Ji

Javascript

<script>

// Javascript implementation of the above approach

    var n;

    // Function to return the count
    // of required strings
    function solve(i , x , dp)
    {
        if (i < 0) {
            return x == 3 ? 1 : 0;
        }
        if (dp[i][x] != -1) {
            return dp[i][x];
        }

        // '0' at ith position
        dp[i][x] = solve(i - 1, 0, dp);

        // '1' at ith position
        dp[i][x] += solve(i - 1, x + 1, dp);
        return dp[i][x];
    }

    // Driver code

        n = 4;
        var dp = Array(n).fill().map(()=>Array(4).fill(0));

        for (i = 0; i < n; i++) {
            for (j = 0; j < 4; j++) {
                dp[i][j] = -1;
            }
        }

        for (i = 0; i < n; i++) {

            // Base condition:
            // 2^(i+1) because of 0 indexing
            dp[i][3] = (1 << (i + 1));
        }
        document.write(solve(n - 1, 0, dp));

// This code contributed by gauravrajput1

</script>

Output:

3

Time complexity: O(N) 
Space Complexity: O(N)