Add solution #779

Author: JoshCrozier

README.md

@@ -589,6 +589,7 @@
 775|[Global and Local Inversions](./0775-global-and-local-inversions.js)|Medium|
 777|[Swap Adjacent in LR String](./0777-swap-adjacent-in-lr-string.js)|Medium|
 778|[Swim in Rising Water](./0778-swim-in-rising-water.js)|Hard|
+779|[K-th Symbol in Grammar](./0779-k-th-symbol-in-grammar.js)|Medium|
 783|[Minimum Distance Between BST Nodes](./0783-minimum-distance-between-bst-nodes.js)|Easy|
 784|[Letter Case Permutation](./0784-letter-case-permutation.js)|Medium|
 790|[Domino and Tromino Tiling](./0790-domino-and-tromino-tiling.js)|Medium|

solutions/0779-k-th-symbol-in-grammar.js

@@ -0,0 +1,31 @@
+/**
+ * 779. K-th Symbol in Grammar
+ * https://leetcode.com/problems/k-th-symbol-in-grammar/
+ * Difficulty: Medium
+ *
+ * We build a table of n rows (1-indexed). We start by writing 0 in the 1st row. Now in every
+ * subsequent row, we look at the previous row and replace each occurrence of 0 with 01, and
+ * each occurrence of 1 with 10.
+ *
+ * - For example, for n = 3, the 1st row is 0, the 2nd row is 01, and the 3rd row is 0110.
+ *
+ * Given two integer n and k, return the kth (1-indexed) symbol in the nth row of a table of n rows.
+ */
+
+/**
+ * @param {number} n
+ * @param {number} k
+ * @return {number}
+ */
+var kthGrammar = function(n, k) {
+  if (n === 1) return 0;
+
+  const length = 1 << (n - 1);
+  const mid = length / 2;
+
+  if (k <= mid) {
+    return kthGrammar(n - 1, k);
+  } else {
+    return 1 - kthGrammar(n - 1, k - mid);
+  }
+};