Add solution #818

Author: JoshCrozier

README.md

@@ -626,6 +626,7 @@
 815|[Bus Routes](./0815-bus-routes.js)|Hard|
 816|[Ambiguous Coordinates](./0816-ambiguous-coordinates.js)|Medium|
 817|[Linked List Components](./0817-linked-list-components.js)|Medium|
+818|[Race Car](./0818-race-car.js)|Hard|
 819|[Most Common Word](./0819-most-common-word.js)|Easy|
 821|[Shortest Distance to a Character](./0821-shortest-distance-to-a-character.js)|Easy|
 824|[Goat Latin](./0824-goat-latin.js)|Easy|

solutions/0818-race-car.js

@@ -0,0 +1,52 @@
+/**
+ * 818. Race Car
+ * https://leetcode.com/problems/race-car/
+ * Difficulty: Hard
+ *
+ * Your car starts at position 0 and speed +1 on an infinite number line. Your car can go into
+ * negative positions. Your car drives automatically according to a sequence of instructions
+ * 'A' (accelerate) and 'R' (reverse):
+ * - When you get an instruction 'A', your car does the following:
+ *   - position += speed
+ *   - speed *= 2
+ * - When you get an instruction 'R', your car does the following:
+ *   - If your speed is positive then speed = -1
+ *   - otherwise speed = 1
+ *
+ * Your position stays the same.
+ *
+ * For example, after commands "AAR", your car goes to positions 0 --> 1 --> 3 --> 3, and your
+ * speed goes to 1 --> 2 --> 4 --> -1.
+ *
+ * Given a target position target, return the length of the shortest sequence of instructions
+ * to get there.
+ */
+
+/**
+ * @param {number} target
+ * @return {number}
+ */
+var racecar = function(target) {
+  const dp = new Array(target + 1).fill(Infinity);
+  dp[0] = 0;
+
+  for (let i = 1; i <= target; i++) {
+    const k = Math.ceil(Math.log2(i + 1));
+
+    if ((1 << k) - 1 === i) {
+      dp[i] = k;
+      continue;
+    }
+
+    dp[i] = k + 1 + dp[(1 << k) - 1 - i];
+
+    for (let j = 0; j < k - 1; j++) {
+      dp[i] = Math.min(
+        dp[i],
+        (k - 1) + 1 + j + 1 + dp[i - ((1 << (k - 1)) - 1) + (1 << j) - 1]
+      );
+    }
+  }
+
+  return dp[target];
+};