p24

inputs/passing/projEuler/p24.ts

@@ -0,0 +1,40 @@
+//! test_output_slow([2,7,8,3,9,1,5,4,6,0])
+
+import range from "../helpers/range.ts";
+
+// TODO: Faster algorithm (it's not necessary to enumerate the permutations).
+
+export default function main() {
+  return range(permsOf([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])).at(999_999);
+}
+
+function* permsOf(nums: number[]) {
+  while (true) {
+    yield nums;
+    nums = nextPerm(nums);
+  }
+}
+
+function nextPerm(nums: number[]) {
+  let i;
+
+  for (i = nums.length - 2; nums[i] >= nums[i + 1]; i--) {
+    if (i === 0) {
+      return nums.reverse();
+    }
+  }
+
+  let j = nums.length - 1;
+
+  while (nums[j] < nums[i]) {
+    j--;
+  }
+
+  [nums[i], nums[j]] = [nums[j], nums[i]];
+
+  const head = nums.slice(0, i + 1);
+  let tail = nums.slice(i + 1);
+  tail.sort((a, b) => a - b);
+
+  return [...head, ...tail];
+}