# Python Program to Count number of binary strings without consecutive 1’s

Given a positive integer N, count all possible distinct binary strings of length N such that there are no consecutive 1’s.

Examples:

Input: N = 2 Output: 3 // The 3 strings are 00, 01, 10 Input: N = 3 Output: 5 // The 5 strings are 000, 001, 010, 100, 101

## Recommended: Please solve it on “__PRACTICE__ ” first, before moving on to the solution.

__PRACTICE__## Python3

`# Python program to count` `# all distinct binary strings` `# without two consecutive 1's` ` ` `def` `countStrings(n):` ` ` ` ` `a ` `=` `[` `0` `for` `i ` `in` `range` `(n)]` ` ` `b ` `=` `[` `0` `for` `i ` `in` `range` `(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` `]` ` ` ` ` `return` `a[n` `-` `1` `] ` `+` `b[n` `-` `1` `]` ` ` `# Driver program to test` `# above functions` ` ` `print` `(countStrings(` `3` `))` ` ` `# This code is contributed` `# by Anant Agarwal.` |

**Output:**

5

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