Add solution #940

Author: JoshCrozier

README.md

@@ -749,6 +749,7 @@
 937|[Reorder Data in Log Files](./solutions/0937-reorder-data-in-log-files.js)|Medium|
 938|[Range Sum of BST](./solutions/0938-range-sum-of-bst.js)|Easy|
 939|[Minimum Area Rectangle](./solutions/0939-minimum-area-rectangle.js)|Medium|
+940|[Distinct Subsequences II](./solutions/0940-distinct-subsequences-ii.js)|Hard|
 966|[Vowel Spellchecker](./solutions/0966-vowel-spellchecker.js)|Medium|
 970|[Powerful Integers](./solutions/0970-powerful-integers.js)|Easy|
 976|[Largest Perimeter Triangle](./solutions/0976-largest-perimeter-triangle.js)|Easy|

solutions/0940-distinct-subsequences-ii.js

@@ -0,0 +1,34 @@
+/**
+ * 940. Distinct Subsequences II
+ * https://leetcode.com/problems/distinct-subsequences-ii/
+ * Difficulty: Hard
+ *
+ * Given a string s, return the number of distinct non-empty subsequences of s. Since the answer
+ * may be very large, return it modulo 109 + 7.
+ *
+ * A subsequence of a string is a new string that is formed from the original string by deleting
+ * some (can be none) of the characters without disturbing the relative positions of the remaining
+ * characters. (i.e., "ace" is a subsequence of "abcde" while "aec" is not.
+ */
+
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var distinctSubseqII = function(s) {
+  const MOD = 1e9 + 7;
+  const lastOccurrence = new Array(26).fill(-1);
+  const dp = new Array(s.length + 1).fill(0);
+  dp[0] = 1;
+
+  for (let i = 0; i < s.length; i++) {
+    const charIndex = s.charCodeAt(i) - 97;
+    dp[i + 1] = (dp[i] * 2) % MOD;
+    if (lastOccurrence[charIndex] !== -1) {
+      dp[i + 1] = (dp[i + 1] - dp[lastOccurrence[charIndex]] + MOD) % MOD;
+    }
+    lastOccurrence[charIndex] = i;
+  }
+
+  return (dp[s.length] - 1 + MOD) % MOD;
+};