Create 1830-minimum-number-of-operations-to-make-string-sorted.js

Author: everthis

1830-minimum-number-of-operations-to-make-string-sorted.js

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+/**
+ * @param {string} s
+ * @return {number}
+ */
+const makeStringSorted = function (s) {
+  const mod = BigInt(10 ** 9 + 7),
+    n = s.length
+  const a = 'a'.charCodeAt(0)
+  let ans = 0n
+  const freq = Array(26).fill(0n)
+  for (let c of s) {
+    freq[c.charCodeAt(0) - a]++
+  }
+  const fact = Array(n + 1).fill(1n)
+  for (let i = 1n; i <= n; i++) {
+    fact[i] = (fact[i - 1n] * i) % mod
+  }
+  let l = n
+  for (let c of s) {
+    l--
+    let t = 0n,
+      rev = 1n
+    for (let i = 0; i < 26; i++) {
+      if (i < c.charCodeAt(0) - a) t += freq[i]
+      rev = (rev * fact[freq[i]]) % mod
+    }
+    ans += ((t * fact[l]) % mod) * modpow(rev, mod - 2n)
+    ans %= mod
+    freq[c.charCodeAt(0) - a]--
+  }
+  return Number(ans)
+  function modpow(b, p) {
+    let ans = 1n
+    for (; p; p >>= 1n) {
+      if (p & 1n) ans = (ans * b) % mod
+      b = (b * b) % mod
+    }
+    return ans
+  }
+}