You can imagine the length as representing the number of slots. Each slot has N possibilities, given that N is the number of elements in your initial list. So given three values [1,2,3]
, you will have a total of 3 x 3 x 3 = 27
permutations.
Here's my attempt. Comments included!
var list = [1,2,3];
var getPermutations = function(list, maxLen) {
// Copy initial values as arrays
var perm = list.map(function(val) {
return [val];
});
// Our permutation generator
var generate = function(perm, maxLen, currLen) {
// Reached desired length
if (currLen === maxLen) {
return perm;
}
// For each existing permutation
for (var i = 0, len = perm.length; i < len; i++) {
var currPerm = perm.shift();
// Create new permutation
for (var k = 0; k < list.length; k++) {
perm.push(currPerm.concat(list[k]));
}
}
// Recurse
return generate(perm, maxLen, currLen + 1);
};
// Start with size 1 because of initial values
return generate(perm, maxLen, 1);
};
var res = getPermutations(list, 3);
console.log(res);
console.log(res.length); // 27