Create 1922-count-good-numbers.js

Author: everthis

1922-count-good-numbers.js

@@ -0,0 +1,23 @@
+/**
+ * @param {number} n
+ * @return {number}
+ */
+const countGoodNumbers = function (n) {
+  n = BigInt(n)
+  const MOD = BigInt(10 ** 9 + 7)
+  let res =
+    quick_pow(5n, (n + 1n) / 2n ) * quick_pow(4n, n / 2n)
+  res %= MOD
+  return res
+
+  function quick_pow(b, m) {
+    let ans = 1n
+    while (m) {
+      if (m % 2n === 1n) ans = (ans * b) % MOD
+      m = m / 2n
+      b = (b * b) % MOD
+    }
+    return ans
+  }
+}
+