Geeksforgeeks

Error

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 ≤ 10^{5}

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Consecutive 1's not allowed

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