Add solution #815

Author: JoshCrozier

Diff for: README.md

@@ -623,6 +623,7 @@
 812|[Largest Triangle Area](./0812-largest-triangle-area.js)|Easy|
 813|[Largest Sum of Averages](./0813-largest-sum-of-averages.js)|Medium|
 814|[Binary Tree Pruning](./0814-binary-tree-pruning.js)|Medium|
+815|[Bus Routes](./0815-bus-routes.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|

Diff for: solutions/0815-bus-routes.js

@@ -0,0 +1,62 @@
+/**
+ * 815. Bus Routes
+ * https://leetcode.com/problems/bus-routes/
+ * Difficulty: Hard
+ *
+ * You are given an array routes representing bus routes where routes[i] is a bus route that
+ * the ith bus repeats forever.
+ *
+ * For example, if routes[0] = [1, 5, 7], this means that the 0th bus travels in the sequence
+ * 1 -> 5 -> 7 -> 1 -> 5 -> 7 -> 1 -> ... forever.
+ *
+ * You will start at the bus stop source (You are not on any bus initially), and you want to
+ * go to the bus stop target. You can travel between bus stops by buses only.
+ *
+ * Return the least number of buses you must take to travel from source to target.
+ *
+ * Return -1 if it is not possible.
+ */
+
+
+/**
+ * @param {number[][]} routes
+ * @param {number} source
+ * @param {number} target
+ * @return {number}
+ */
+var numBusesToDestination = function(routes, source, target) {
+  if (source === target) return 0;
+
+  const stopToBuses = new Map();
+
+  for (let busId = 0; busId < routes.length; busId++) {
+    for (const stop of routes[busId]) {
+      if (!stopToBuses.has(stop)) {
+        stopToBuses.set(stop, []);
+      }
+      stopToBuses.get(stop).push(busId);
+    }
+  }
+  if (!stopToBuses.has(source) || !stopToBuses.has(target)) return -1;
+
+  const visitedBuses = new Set();
+  const visitedStops = new Set([source]);
+  const queue = [[source, 0]];
+
+  while (queue.length > 0) {
+    const [currentStop, busCount] = queue.shift();
+    if (currentStop === target) return busCount;
+    for (const busId of stopToBuses.get(currentStop)) {
+      if (visitedBuses.has(busId)) continue;
+      visitedBuses.add(busId);
+      for (const nextStop of routes[busId]) {
+        if (visitedStops.has(nextStop)) continue;
+        visitedStops.add(nextStop);
+        queue.push([nextStop, busCount + 1]);
+        if (nextStop === target) return busCount + 1;
+      }
+    }
+  }
+
+  return -1;
+};