Add solution #1643

Author: JoshCrozier

README.md

@@ -1,4 +1,4 @@
-# 1,444 LeetCode solutions in JavaScript
+# 1,445 LeetCode solutions in JavaScript
 
 [https://leetcodejavascript.com](https://leetcodejavascript.com)
 
@@ -1267,6 +1267,7 @@
 1640|[Check Array Formation Through Concatenation](./solutions/1640-check-array-formation-through-concatenation.js)|Easy|
 1641|[Count Sorted Vowel Strings](./solutions/1641-count-sorted-vowel-strings.js)|Medium|
 1642|[Furthest Building You Can Reach](./solutions/1642-furthest-building-you-can-reach.js)|Medium|
+1643|[Kth Smallest Instructions](./solutions/1643-kth-smallest-instructions.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/1643-kth-smallest-instructions.js

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+/**
+ * 1643. Kth Smallest Instructions
+ * https://leetcode.com/problems/kth-smallest-instructions/
+ * Difficulty: Hard
+ *
+ * Bob is standing at cell (0, 0), and he wants to reach destination: (row, column). He can only
+ * travel right and down. You are going to help Bob by providing instructions for him to reach
+ * destination.
+ *
+ * The instructions are represented as a string, where each character is either:
+ * - 'H', meaning move horizontally (go right), or
+ * - 'V', meaning move vertically (go down).
+ *
+ * Multiple instructions will lead Bob to destination. For example, if destination is (2, 3),
+ * both "HHHVV" and "HVHVH" are valid instructions.
+ *
+ * However, Bob is very picky. Bob has a lucky number k, and he wants the kth lexicographically
+ * smallest instructions that will lead him to destination. k is 1-indexed.
+ *
+ * Given an integer array destination and an integer k, return the kth lexicographically
+ * smallest instructions that will take Bob to destination.
+ */
+
+/**
+ * @param {number[]} destination
+ * @param {number} k
+ * @return {string}
+ */
+var kthSmallestPath = function(destination, k) {
+  let verticalMoves = destination[0];
+  let horizontalMoves = destination[1];
+  const totalMoves = verticalMoves + horizontalMoves;
+  let path = '';
+
+  for (let i = 0; i < totalMoves; i++) {
+    if (horizontalMoves === 0) {
+      path += 'V';
+      verticalMoves--;
+      continue;
+    }
+    const combinations = calculateCombinations(
+      verticalMoves + horizontalMoves - 1,
+      horizontalMoves - 1
+    );
+    if (k <= combinations) {
+      path += 'H';
+      horizontalMoves--;
+    } else {
+      path += 'V';
+      verticalMoves--;
+      k -= combinations;
+    }
+  }
+
+  return path;
+};
+
+function calculateCombinations(n, r) {
+  if (r < 0 || r > n) return 0;
+  let result = 1;
+  for (let i = 1; i <= r; i++) {
+    result = result * (n - i + 1) / i;
+  }
+  return Math.floor(result);
+}