Create 1136-parallel-courses.js

Author: everthis

1136-parallel-courses.js

@@ -0,0 +1,35 @@
+/**
+ * @param {number} N
+ * @param {number[][]} relations
+ * @return {number}
+ */
+const minimumSemesters = function (N, relations) {
+  const g = new Map() // key: prerequisite, value: course list.
+  const inDegree = new Array(N + 1).fill(0) // inDegree[i]: number of prerequisites for i.
+  for (let r of relations) {
+    if (!g.has(r[0])) g.set(r[0], [])
+    g.get(r[0]).push(r[1]) // construct graph.
+    ++inDegree[r[1]] // count prerequisites for r[1].
+  }
+  const q = [] // save current 0 in-degree vertices.
+  for (let i = 1; i <= N; ++i) if (inDegree[i] === 0) q.push(i)
+  let semester = 0
+  while (q.length) {
+    // BFS traverse all currently 0 in degree vertices.
+    for (let sz = q.length; sz > 0; --sz) {
+      // sz is the search breadth.
+      const c = q.shift()
+      --N
+      // c's in-degree is currently 0, but it is not a prerequisite of anyone else.
+      if (!g.has(c)) continue
+      const tmp = g.get(c)
+      g.delete(c)
+      for (let course of tmp)
+        if (--inDegree[course] === 0)
+          // decrease the in-degree of course's neighbors.
+          q.push(course) // add current 0 in-degree vertex into Queue.
+    }
+    ++semester // need one more semester.
+  }
+  return N === 0 ? semester : -1
+}