Create 828-unique-letter-string.js

Author: everthis

828-unique-letter-string.js

@@ -0,0 +1,22 @@
+/**
+ * @param {string} S
+ * @return {number}
+ */
+const uniqueLetterString = function(S) {
+  const len = S.length
+  if (len === 0) return 0
+  if (len === 1) return 1
+  let count = 0
+  let lastP = new Array(26).fill(0)
+  let lastLastP = new Array(26).fill(0)
+  let pre = 0
+  for (let i = 0; i < len; i++) {
+    let idx = S.charCodeAt(i) - 'A'.charCodeAt(0)
+    pre += i - lastP[idx] + 1
+    pre -= lastP[idx] - lastLastP[idx]
+    count += pre
+    lastLastP[idx] = lastP[idx]
+    lastP[idx] = i + 1
+  }
+  return count % 1000000007
+}