Given two integers n and k, count the number of binary strings of length n with k as number of times adjacent 1’s appear.

**Examples:**

Input : n = 5, k = 2 Output : 6 Explanation: Binary strings of length 5 in which k number of times two adjacent set bits appear. 001110111011100110111011111101 Input : n = 4, k = 1 Output : 3 Explanation: Binary strings of length 3 in which k number of times two adjacent set bits appear. 00111100 0110

Lets try writing the recursive function for the above problem statement:

1) n = 1, only two binary strings exist with length 1, not having any adjacent 1’s

String 1 : “0”

String 2 : “1”

2) For all n > 1 and all k, two cases arise

a) Strings ending with 0 : String of length n can be created by appending 0 to all **strings of length n-1 having k times two adjacent 1’s ending with both 0 and 1** (Having 0 at n’th position will not change the count of adjacent 1’s).

b) Strings ending with 1 : String of length n can be created by appending 1 to all strings of **length n-1 having k times adjacent 1’s and ending with 0** and to all strings of **length n-1 having k-1 adjacent 1’s and ending with 1**.

Example: let s = 011 i.e. a string ending with 1 having adjacent count as 1. Adding 1 to it, s = 0111 increase the count of adjacent 1.

Let there be an array dp[i][j][2] wheredp[i][j][0]denotes number of binary strings with length i having j number of two adjacent 1's and ending with 0. Similarlydp[i][j][1]denotes the same binary strings with length i and j adjacent 1's but ending with 1. Then: dp[1][0][0] = 1 and dp[1][0][1] = 1 For all other i and j, dp[i][j][0] = dp[i-1][j][0] + dp[i-1][j][1] dp[i][j][1] = dp[i-1][j][0] + dp[i-1][j-1][1] Then, output dp[n][k][0] + dp[n][k][1]

## C++

`// C++ program to count number of binary strings ` `// with k times appearing consecutive 1's. ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `int` `countStrings(` `int` `n, ` `int` `k) ` `{ ` ` ` `// dp[i][j][0] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 0. ` ` ` `// dp[i][j][1] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 1. ` ` ` `int` `dp[n + 1][k + 1][2]; ` ` ` `memset` `(dp, 0, ` `sizeof` `(dp)); ` ` ` ` ` `// If n = 1 and k = 0. ` ` ` `dp[1][0][0] = 1; ` ` ` `dp[1][0][1] = 1; ` ` ` ` ` `for` `(` `int` `i = 2; i <= n; i++) { ` ` ` ` ` `// number of adjacent 1's can not exceed i-1 ` ` ` `for` `(` `int` `j = 0; j <= k; j++) { ` ` ` `dp[i][j][0] = dp[i - 1][j][0] + dp[i - 1][j][1]; ` ` ` `dp[i][j][1] = dp[i - 1][j][0]; ` ` ` ` ` `if` `(j - 1 >= 0) ` ` ` `dp[i][j][1] += dp[i - 1][j - 1][1]; ` ` ` `} ` ` ` `} ` ` ` ` ` `return` `dp[n][k][0] + dp[n][k][1]; ` `} ` ` ` `// Driver code ` `int` `main() ` `{ ` ` ` `int` `n = 5, k = 2; ` ` ` `cout << countStrings(n, k); ` ` ` `return` `0; ` `}` |

## Java

`// Java program to count number of binary strings ` `// with k times appearing consecutive 1's. ` `class` `GFG { ` ` ` ` ` `static` `int` `countStrings(` `int` `n, ` `int` `k) ` ` ` `{ ` ` ` `// dp[i][j][0] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 0. ` ` ` `// dp[i][j][1] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 1. ` ` ` `int` `dp[][][] = ` `new` `int` `[n + ` `1` `][k + ` `1` `][` `2` `]; ` ` ` ` ` `// If n = 1 and k = 0. ` ` ` `dp[` `1` `][` `0` `][` `0` `] = ` `1` `; ` ` ` `dp[` `1` `][` `0` `][` `1` `] = ` `1` `; ` ` ` ` ` `for` `(` `int` `i = ` `2` `; i <= n; i++) { ` ` ` ` ` `// number of adjacent 1's can not exceed i-1 ` ` ` `for` `(` `int` `j = ` `0` `; j < i && j < k + ` `1` `; j++) { ` ` ` `dp[i][j][` `0` `] = dp[i - ` `1` `][j][` `0` `] + dp[i - ` `1` `][j][` `1` `]; ` ` ` `dp[i][j][` `1` `] = dp[i - ` `1` `][j][` `0` `]; ` ` ` ` ` `if` `(j - ` `1` `>= ` `0` `) { ` ` ` `dp[i][j][` `1` `] += dp[i - ` `1` `][j - ` `1` `][` `1` `]; ` ` ` `} ` ` ` `} ` ` ` `} ` ` ` ` ` `return` `dp[n][k][` `0` `] + dp[n][k][` `1` `]; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main(String[] args) ` ` ` `{ ` ` ` `int` `n = ` `5` `, k = ` `2` `; ` ` ` `System.out.println(countStrings(n, k)); ` ` ` `} ` `} ` ` ` `// This code has been contributed by 29AjayKumar ` |

## Python3

`# Python3 program to count number of ` `# binary strings with k times appearing ` `# consecutive 1's. ` `def` `countStrings(n, k): ` ` ` ` ` `# dp[i][j][0] stores count of binary ` ` ` `# strings of length i with j consecutive ` ` ` `# 1's and ending at 0. ` ` ` `# dp[i][j][1] stores count of binary ` ` ` `# strings of length i with j consecutive ` ` ` `# 1's and ending at 1. ` ` ` `dp ` `=` `[[[` `0` `, ` `0` `] ` `for` `__ ` `in` `range` `(k ` `+` `1` `)] ` ` ` `for` `_ ` `in` `range` `(n ` `+` `1` `)] ` ` ` ` ` `# If n = 1 and k = 0. ` ` ` `dp[` `1` `][` `0` `][` `0` `] ` `=` `1` ` ` `dp[` `1` `][` `0` `][` `1` `] ` `=` `1` ` ` ` ` `for` `i ` `in` `range` `(` `2` `, n ` `+` `1` `): ` ` ` ` ` `# number of adjacent 1's can not exceed i-1 ` ` ` `for` `j ` `in` `range` `(k ` `+` `1` `): ` ` ` `dp[i][j][` `0` `] ` `=` `(dp[i ` `-` `1` `][j][` `0` `] ` `+` ` ` `dp[i ` `-` `1` `][j][` `1` `]) ` ` ` `dp[i][j][` `1` `] ` `=` `dp[i ` `-` `1` `][j][` `0` `] ` ` ` `if` `j >` `=` `1` `: ` ` ` `dp[i][j][` `1` `] ` `+` `=` `dp[i ` `-` `1` `][j ` `-` `1` `][` `1` `] ` ` ` ` ` `return` `dp[n][k][` `0` `] ` `+` `dp[n][k][` `1` `] ` ` ` `# Driver Code ` `if` `__name__ ` `=` `=` `'__main__'` `: ` ` ` `n ` `=` `5` ` ` `k ` `=` `2` ` ` `print` `(countStrings(n, k)) ` ` ` `# This code is contributed by vibhu4agarwal ` |

## C#

`// C# program to count number of binary strings ` `// with k times appearing consecutive 1's. ` `using` `System; ` ` ` `class` `GFG { ` ` ` ` ` `static` `int` `countStrings(` `int` `n, ` `int` `k) ` ` ` `{ ` ` ` `// dp[i][j][0] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 0. ` ` ` `// dp[i][j][1] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 1. ` ` ` `int` `[,, ] dp = ` `new` `int` `[n + 1, k + 1, 2]; ` ` ` ` ` `// If n = 1 and k = 0. ` ` ` `dp[1, 0, 0] = 1; ` ` ` `dp[1, 0, 1] = 1; ` ` ` ` ` `for` `(` `int` `i = 2; i <= n; i++) { ` ` ` ` ` `// number of adjacent 1's can not exceed i-1 ` ` ` `for` `(` `int` `j = 0; j < i && j < k + 1; j++) { ` ` ` `dp[i, j, 0] = dp[i - 1, j, 0] + dp[i - 1, j, 1]; ` ` ` `dp[i, j, 1] = dp[i - 1, j, 0]; ` ` ` ` ` `if` `(j - 1 >= 0) { ` ` ` `dp[i, j, 1] += dp[i - 1, j - 1, 1]; ` ` ` `} ` ` ` `} ` ` ` `} ` ` ` ` ` `return` `dp[n, k, 0] + dp[n, k, 1]; ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `Main(String[] args) ` ` ` `{ ` ` ` `int` `n = 5, k = 2; ` ` ` `Console.WriteLine(countStrings(n, k)); ` ` ` `} ` `} ` ` ` `// This code contributed by Rajput-Ji ` |

## PHP

`<?php ` `// PHP program to count number of binary strings ` `// with k times appearing consecutive 1's. ` ` ` `function` `countStrings(` `$n` `, ` `$k` `) ` `{ ` ` ` `// dp[i][j][0] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 0. ` ` ` `// dp[i][j][1] stores count of binary ` ` ` `// strings of length i with j consecutive ` ` ` `// 1's and ending at 1. ` ` ` `$dp` `=` `array_fill` `(0, ` `$n` `+ 1, ` `array_fill` `(0, ` `$k` `+ 1, ` `array_fill` `(0, 2, 0))); ` ` ` ` ` `// If n = 1 and k = 0. ` ` ` `$dp` `[1][0][0] = 1; ` ` ` `$dp` `[1][0][1] = 1; ` ` ` ` ` `for` `(` `$i` `= 2; ` `$i` `<= ` `$n` `; ` `$i` `++) ` ` ` `{ ` ` ` `// number of adjacent 1's can not exceed i-1 ` ` ` `for` `(` `$j` `= 0; ` `$j` `< ` `$i` `; ` `$j` `++) ` ` ` `{ ` ` ` `if` `(isset(` `$dp` `[` `$i` `][` `$j` `][0])||isset(` `$dp` `[` `$i` `][` `$j` `][1])){ ` ` ` `$dp` `[` `$i` `][` `$j` `][0] = ` `$dp` `[` `$i` `- 1][` `$j` `][0] + ` `$dp` `[` `$i` `- 1][` `$j` `][1]; ` ` ` `$dp` `[` `$i` `][` `$j` `][1] = ` `$dp` `[` `$i` `- 1][` `$j` `][0]; ` ` ` `} ` ` ` `if` `(` `$j` `- 1 >= 0 && isset(` `$dp` `[` `$i` `][` `$j` `][1])) ` ` ` `$dp` `[` `$i` `][` `$j` `][1] += ` `$dp` `[` `$i` `- 1][` `$j` `- 1][1]; ` ` ` `} ` ` ` `} ` ` ` ` ` `return` `$dp` `[` `$n` `][` `$k` `][0] + ` `$dp` `[` `$n` `][` `$k` `][1]; ` `} ` ` ` `// Driver code ` `$n` `=5; ` `$k` `=2; ` `echo` `countStrings(` `$n` `, ` `$k` `); ` ` ` `// This code is contributed by mits ` `?> ` |

**Output:**

6

**Time Complexity :** O(n^{2})

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