Add solution #1930

Author: JoshCrozier

README.md

@@ -623,6 +623,7 @@
 1920|[Build Array from Permutation](./1920-build-array-from-permutation.js)|Easy|
 1926|[Nearest Exit from Entrance in Maze](./1926-nearest-exit-from-entrance-in-maze.js)|Medium|
 1929|[Concatenation of Array](./1929-concatenation-of-array.js)|Easy|
+1930|[Unique Length-3 Palindromic Subsequences](./1930-unique-length-3-palindromic-subsequences.js)|Medium|
 1935|[Maximum Number of Words You Can Type](./1935-maximum-number-of-words-you-can-type.js)|Easy|
 1980|[Find Unique Binary String](./1980-find-unique-binary-string.js)|Medium|
 1985|[Find the Kth Largest Integer in the Array](./1985-find-the-kth-largest-integer-in-the-array.js)|Medium|

solutions/1930-unique-length-3-palindromic-subsequences.js

@@ -0,0 +1,38 @@
+/**
+ * 1930. Unique Length-3 Palindromic Subsequences
+ * https://leetcode.com/problems/unique-length-3-palindromic-subsequences/
+ * Difficulty: Medium
+ *
+ * Given a string s, return the number of unique palindromes of length three that are a
+ * subsequence of s.
+ *
+ * Note that even if there are multiple ways to obtain the same subsequence, it is still
+ * only counted once.
+ *
+ * A palindrome is a string that reads the same forwards and backwards.
+ *
+ * A subsequence of a string is a new string generated from the original string with some
+ * characters (can be none) deleted without changing the relative order of the remaining
+ * characters.
+ *
+ * For example, "ace" is a subsequence of "abcde".
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var countPalindromicSubsequence = function(s) {
+  let result = 0;
+
+  for (let i = 0; i < 26; ++i) {
+    const char = String.fromCharCode(i + 97);
+    const left = s.indexOf(char);
+    const right = s.lastIndexOf(char);
+    if (left !== -1 && right !== -1 && left < right) {
+      result += new Set(s.substring(left + 1, right)).size;
+    }
+  }
+
+  return result;
+};