조합 구하기

function string_recurse(active, rest) {
    if (rest.length == 0) {
        console.log(active);
    } else {
        string_recurse(active + rest.charAt(0), rest.substring(1, rest.length));
        string_recurse(active, rest.substring(1, rest.length));
    }
}
string_recurse("", "abc");

function k_combinations(set, k) {
  var i, j, combs, head, tailcombs;

  if (k > set.length || k <= 0) {
    return [];
  }

  if (k == set.length) {
    return [set];
  }

  if (k == 1) {
    combs = [];
    for (i = 0; i < set.length; i++) {
      combs.push([set[i]]);
    }
    return combs;
  }

  combs = [];
  for (i = 0; i < set.length - k + 1; i++) {
    head = set.slice(i, i + 1);
    tailcombs = k_combinations(set.slice(i + 1), k - 1);
    for (j = 0; j < tailcombs.length; j++) {
      combs.push(head.concat(tailcombs[j]));
    }
  }
  return combs;
}

function combinations(set) {
  var k, i, combs, k_combs;
  combs = [];

  for (k = 1; k <= set.length; k++) {
    k_combs = k_combinations(set, k);
    for (i = 0; i < k_combs.length; i++) {
      combs.push(k_combs[i]);
    }
  }
  return combs;
}