Add solution #1987

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,671 LeetCode solutions in JavaScript
+# 1,672 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1525,6 +1525,7 @@
 1984|[Minimum Difference Between Highest and Lowest of K Scores](./solutions/1984-minimum-difference-between-highest-and-lowest-of-k-scores.js)|Easy|
 1985|[Find the Kth Largest Integer in the Array](./solutions/1985-find-the-kth-largest-integer-in-the-array.js)|Medium|
 1986|[Minimum Number of Work Sessions to Finish the Tasks](./solutions/1986-minimum-number-of-work-sessions-to-finish-the-tasks.js)|Medium|
+1987|[Number of Unique Good Subsequences](./solutions/1987-number-of-unique-good-subsequences.js)|Hard|
 1996|[The Number of Weak Characters in the Game](./solutions/1996-the-number-of-weak-characters-in-the-game.js)|Medium|
 2000|[Reverse Prefix of Word](./solutions/2000-reverse-prefix-of-word.js)|Easy|
 2011|[Final Value of Variable After Performing Operations](./solutions/2011-final-value-of-variable-after-performing-operations.js)|Easy|

solutions/1987-number-of-unique-good-subsequences.js

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+/**
+ * 1987. Number of Unique Good Subsequences
+ * https://leetcode.com/problems/number-of-unique-good-subsequences/
+ * Difficulty: Hard
+ *
+ * You are given a binary string binary. A subsequence of binary is considered good if it is not
+ * empty and has no leading zeros (with the exception of "0").
+ *
+ * Find the number of unique good subsequences of binary.
+ * - For example, if binary = "001", then all the good subsequences are ["0", "0", "1"], so the
+ *   unique good subsequences are "0" and "1". Note that subsequences "00", "01", and "001" are
+ *   not good because they have leading zeros.
+ *
+ * Return the number of unique good subsequences of binary. Since the answer may be very large,
+ * return it modulo 109 + 7.
+ *
+ * A subsequence is a sequence that can be derived from another sequence by deleting some or no
+ * elements without changing the order of the remaining elements.
+ */
+
+/**
+ * @param {string} binary
+ * @return {number}
+ */
+var numberOfUniqueGoodSubsequences = function(binary) {
+  const MOD = 1e9 + 7;
+  let endsWithZero = 0;
+  let endsWithOne = 0;
+  let hasZero = false;
+
+  for (const digit of binary) {
+    if (digit === '0') {
+      hasZero = true;
+      endsWithZero = (endsWithZero + endsWithOne) % MOD;
+    } else {
+      endsWithOne = (endsWithOne + endsWithZero + 1) % MOD;
+    }
+  }
+
+  return (endsWithZero + endsWithOne + (hasZero ? 1 : 0)) % MOD;
+};