Consecutive 1's not allowed
Medium Accuracy: 27.07% Submissions: 1037 Points: 4

Given a positive integer N, count all possible distinct binary strings of length N such that there are no consecutive 1’s. Output your answer modulo 10^9 + 7.

Example 1:

Input:
N = 3
Output: 5
Explanation: 5 strings are (000,
001, 010, 100, 101).

Example 2:

Input:
N = 2
Output: 3
Explanation: 3 strings are
(00,01,10).

Your Task:
Complete the function countStrings() which takes single integer n, as input parameters and returns an integer denoting the answer. You don't to print answer or take inputs. 

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(N)

Constraints:
1 ≤ N ≤ 105

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