Add solution #1639

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,440 LeetCode solutions in JavaScript
+# 1,441 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1263,6 +1263,7 @@
 1636|[Sort Array by Increasing Frequency](./solutions/1636-sort-array-by-increasing-frequency.js)|Easy|
 1637|[Widest Vertical Area Between Two Points Containing No Points](./solutions/1637-widest-vertical-area-between-two-points-containing-no-points.js)|Easy|
 1638|[Count Substrings That Differ by One Character](./solutions/1638-count-substrings-that-differ-by-one-character.js)|Medium|
+1639|[Number of Ways to Form a Target String Given a Dictionary](./solutions/1639-number-of-ways-to-form-a-target-string-given-a-dictionary.js)|Hard|
 1657|[Determine if Two Strings Are Close](./solutions/1657-determine-if-two-strings-are-close.js)|Medium|
 1668|[Maximum Repeating Substring](./solutions/1668-maximum-repeating-substring.js)|Easy|
 1669|[Merge In Between Linked Lists](./solutions/1669-merge-in-between-linked-lists.js)|Medium|

solutions/1639-number-of-ways-to-form-a-target-string-given-a-dictionary.js

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+/**
+ * 1639. Number of Ways to Form a Target String Given a Dictionary
+ * https://leetcode.com/problems/number-of-ways-to-form-a-target-string-given-a-dictionary/
+ * Difficulty: Hard
+ *
+ * You are given a list of strings of the same length words and a string target.
+ *
+ * Your task is to form target using the given words under the following rules:
+ * - target should be formed from left to right.
+ * - To form the ith character (0-indexed) of target, you can choose the kth character of the jth
+ *   string in words if target[i] = words[j][k].
+ * - Once you use the kth character of the jth string of words, you can no longer use the xth
+ *   character of any string in words where x <= k. In other words, all characters to the left of
+ *   or at index k become unusuable for every string.
+ * - Repeat the process until you form the string target.
+ *
+ * Notice that you can use multiple characters from the same string in words provided the
+ * conditions above are met.
+ *
+ * Return the number of ways to form target from words. Since the answer may be too large, return
+ * it modulo 109 + 7.
+ */
+
+/**
+ * @param {string[]} words
+ * @param {string} target
+ * @return {number}
+ */
+var numWays = function(words, target) {
+  const MOD = 1e9 + 7;
+  const wordLength = words[0].length;
+  const targetLength = target.length;
+  const charCounts = Array(wordLength).fill().map(() => Array(26).fill(0));
+
+  for (const word of words) {
+    for (let i = 0; i < wordLength; i++) {
+      charCounts[i][word.charCodeAt(i) - 97]++;
+    }
+  }
+
+  const dp = Array(targetLength + 1).fill().map(() => Array(wordLength + 1).fill(0));
+  dp[0][0] = 1;
+
+  for (let i = 0; i <= targetLength; i++) {
+    for (let j = 0; j < wordLength; j++) {
+      if (i < targetLength) {
+        const charIndex = target.charCodeAt(i) - 97;
+        dp[i + 1][j + 1] = (dp[i + 1][j + 1] + dp[i][j] * charCounts[j][charIndex]) % MOD;
+      }
+      dp[i][j + 1] = (dp[i][j + 1] + dp[i][j]) % MOD;
+    }
+  }
+
+  return dp[targetLength][wordLength];
+};