Add solution #756

Author: JoshCrozier

README.md

@@ -573,6 +573,7 @@
 752|[Open the Lock](./0752-open-the-lock.js)|Medium|
 753|[Cracking the Safe](./0753-cracking-the-safe.js)|Hard|
 754|[Reach a Number](./0754-reach-a-number.js)|Medium|
+756|[Pyramid Transition Matrix](./0756-pyramid-transition-matrix.js)|Medium|
 762|[Prime Number of Set Bits in Binary Representation](./0762-prime-number-of-set-bits-in-binary-representation.js)|Easy|
 763|[Partition Labels](./0763-partition-labels.js)|Medium|
 783|[Minimum Distance Between BST Nodes](./0783-minimum-distance-between-bst-nodes.js)|Easy|

solutions/0756-pyramid-transition-matrix.js

@@ -0,0 +1,53 @@
+/**
+ * 756. Pyramid Transition Matrix
+ * https://leetcode.com/problems/pyramid-transition-matrix/
+ * Difficulty: Medium
+ *
+ * You are stacking blocks to form a pyramid. Each block has a color, which is represented by
+ * a single letter. Each row of blocks contains one less block than the row beneath it and is
+ * centered on top.
+ *
+ * To make the pyramid aesthetically pleasing, there are only specific triangular patterns that
+ * are allowed. A triangular pattern consists of a single block stacked on top of two blocks.
+ * The patterns are given as a list of three-letter strings allowed, where the first two characters
+ * of a pattern represent the left and right bottom blocks respectively, and the third character
+ * is the top block.
+ *
+ * For example, "ABC" represents a triangular pattern with a 'C' block stacked on top of an 'A'
+ * (left) and 'B' (right) block. Note that this is different from "BAC" where 'B' is on the left
+ * bottom and 'A' is on the right bottom.
+ *
+ * You start with a bottom row of blocks bottom, given as a single string, that you must use as
+ * the base of the pyramid.
+ *
+ * Given bottom and allowed, return true if you can build the pyramid all the way to the top such
+ * that every triangular pattern in the pyramid is in allowed, or false otherwise.
+ */
+
+/**
+ * @param {string} bottom
+ * @param {string[]} allowed
+ * @return {boolean}
+ */
+var pyramidTransition = function(bottom, allowed) {
+  const transitions = new Map();
+  for (const [left, right, top] of allowed) {
+    const key = left + right;
+    transitions.set(key, (transitions.get(key) || '') + top);
+  }
+
+  function canBuild(row, nextRow = '') {
+    if (row.length === 1) return true;
+    if (nextRow.length === row.length - 1) return canBuild(nextRow);
+
+    const pair = row.slice(nextRow.length, nextRow.length + 2);
+    const options = transitions.get(pair) || '';
+
+    for (const top of options) {
+      if (canBuild(row, nextRow + top)) return true;
+    }
+    return false;
+  }
+
+  return canBuild(bottom);
+};