Challenge>fibonacciIndex

Consider the Fibonacci sequence: 0 1 1 2 3 5 8 13 21 ... We can see that 7 is the smallest 0-based index k for which F(k) has exactly 2 decimal digits. What is the smallest index k for which F(k) has exactly n decimal digits? Example For n = 1, the output should be fibonacciIndex(n) = 0; For n = 2, the output should be fibonacciIndex(n) = 7.

code>>

int fibonacciIndex(int n) {

if (n == 1) return 0;

int f = 0, s = 1, t = 1, idx = 1;

while (std::to_string(t).size() != n)

{

t = f + s;

f = s;

s = t;

idx++;

}

return idx;

}