Add solution #838

Author: JoshCrozier

README.md

@@ -645,6 +645,7 @@
 835|[Image Overlap](./0835-image-overlap.js)|Medium|
 836|[Rectangle Overlap](./0836-rectangle-overlap.js)|Easy|
 837|[New 21 Game](./0837-new-21-game.js)|Medium|
+838|[Push Dominoes](./0838-push-dominoes.js)|Medium|
 841|[Keys and Rooms](./0841-keys-and-rooms.js)|Medium|
 844|[Backspace String Compare](./0844-backspace-string-compare.js)|Easy|
 846|[Hand of Straights](./0846-hand-of-straights.js)|Medium|

solutions/0838-push-dominoes.js

@@ -0,0 +1,63 @@
+/**
+ * 838. Push Dominoes
+ * https://leetcode.com/problems/push-dominoes/
+ * Difficulty: Medium
+ *
+ * There are n dominoes in a line, and we place each domino vertically upright. In the beginning,
+ * we simultaneously push some of the dominoes either to the left or to the right.
+ *
+ * After each second, each domino that is falling to the left pushes the adjacent domino on the
+ * left. Similarly, the dominoes falling to the right push their adjacent dominoes standing on
+ * the right.
+ *
+ * When a vertical domino has dominoes falling on it from both sides, it stays still due to the
+ * balance of the forces.
+ *
+ * For the purposes of this question, we will consider that a falling domino expends no additional
+ * force to a falling or already fallen domino.
+ *
+ * You are given a string dominoes representing the initial state where:
+ * - dominoes[i] = 'L', if the ith domino has been pushed to the left,
+ * - dominoes[i] = 'R', if the ith domino has been pushed to the right, and
+ * - dominoes[i] = '.', if the ith domino has not been pushed.
+ *
+ * Return a string representing the final state.
+ */
+
+/**
+ * @param {string} dominoes
+ * @return {string}
+ */
+var pushDominoes = function(dominoes) {
+  const forces = new Array(dominoes.length).fill(0);
+
+  let force = 0;
+  for (let i = 0; i < dominoes.length; i++) {
+    if (dominoes[i] === 'R') {
+      force = dominoes.length;
+    } else if (dominoes[i] === 'L') {
+      force = 0;
+    } else {
+      force = Math.max(force - 1, 0);
+    }
+    forces[i] += force;
+  }
+
+  force = 0;
+  for (let i = dominoes.length - 1; i >= 0; i--) {
+    if (dominoes[i] === 'L') {
+      force = dominoes.length;
+    } else if (dominoes[i] === 'R') {
+      force = 0;
+    } else {
+      force = Math.max(force - 1, 0);
+    }
+    forces[i] -= force;
+  }
+
+  return forces.map(f => {
+    if (f > 0) return 'R';
+    if (f < 0) return 'L';
+    return '.';
+  }).join('');
+};