2.2 KiB
2.2 KiB
id | title | challengeType | forumTopicId | dashedName |
---|---|---|---|---|
5900f3951000cf542c50fea8 | Problem 41: Pandigital prime | 5 | 302078 | problem-41-pandigital-prime |
--description--
We shall say that an n
-digit number is pandigital if it makes use of all the digits 1 to n
exactly once. For example, 2143 is a 4-digit pandigital and is also prime.
What is the largest n
-length digit pandigital prime that exists?
--hints--
pandigitalPrime(4)
should return a number.
assert(typeof pandigitalPrime(4) === 'number');
pandigitalPrime(4)
should return 4231.
assert(pandigitalPrime(4) == 4231);
pandigitalPrime(7)
should return 7652413.
assert(pandigitalPrime(7) == 7652413);
--seed--
--seed-contents--
function pandigitalPrime(n) {
return n;
}
pandigitalPrime(7);
--solutions--
function pandigitalPrime(n) {
function isPrime(num) {
for (let i = 2, s = Math.sqrt(num); i <= s; i++) {
if (num % i === 0) {
return false;
}
}
return num !== 1;
}
function getPermutations(n) {
if (n === 1) {
permutations.push(digitsArr.join(''));
} else {
for (let i = 0; i < n - 1; i++) {
getPermutations(n - 1);
// swap(n % 2 === 0 ? i : 0, n - 1);
if (n % 2 === 0) {
swap(i, n - 1);
} else {
swap(0, n - 1);
}
}
getPermutations(n - 1);
}
}
function swap(x, y) {
let temp = digitsArr[x];
digitsArr[x] = digitsArr[y];
digitsArr[y] = temp;
}
let max = 0;
let permutations = [];
let digitsArr;
let pandigitalNum = '';
for (let max = n; max > 0; max--) {
pandigitalNum += max;
}
for (let i = 0; i < pandigitalNum.length; i++) {
if (max > 0) {
break;
} else {
permutations = [];
const currMax = pandigitalNum.slice(i);
digitsArr = currMax.split('');
getPermutations(digitsArr.length);
// sort permutations in descending order
permutations.sort(function(a, b) {
return b - a;
});
for (let perm of permutations) {
const thisPerm = parseInt(perm);
if (isPrime(thisPerm)) {
max = thisPerm;
break;
}
}
}
}
return max;
}