1.9 KiB
1.9 KiB
id | title | challengeType | forumTopicId | dashedName |
---|---|---|---|---|
5900f3851000cf542c50fe98 | Problem 25: 1000-digit Fibonacci number | 5 | 301897 | problem-25-1000-digit-fibonacci-number |
--description--
The Fibonacci sequence is defined by the recurrence relation:
Fn = Fn−1 + Fn−2, where F1 = 1 and F2 = 1.
Hence the first 12 terms will be:
F1 = 1
F2 = 1
F3 = 2
F4 = 3
F5 = 5
F6 = 8
F7 = 13
F8 = 21
F9 = 34
F10 = 55
F11 = 89
F12 = 144
The 12th term, F12, is the first term to contain three digits.
What is the index of the first term in the Fibonacci sequence to contain n
digits?
--hints--
digitFibonacci(5)
should return a number.
assert(typeof digitFibonacci(5) === 'number');
digitFibonacci(5)
should return 21.
assert.strictEqual(digitFibonacci(5), 21);
digitFibonacci(10)
should return 45.
assert.strictEqual(digitFibonacci(10), 45);
digitFibonacci(15)
should return 69.
assert.strictEqual(digitFibonacci(15), 69);
digitFibonacci(20)
should return 93.
assert.strictEqual(digitFibonacci(20), 93);
--seed--
--seed-contents--
function digitFibonacci(n) {
return n;
}
digitFibonacci(20);
--solutions--
const digitFibonacci = (n) => {
const digits = (num) => {
return num.toString().length;
};
let f1 = 1;
let f2 = 1;
let index = 3;
while (true) {
let fn = f1 + f2;
if (digits(fn) === n) return index;
[f1, f2] = [f2, fn];
index++;
}
};