topological sort

Author: dingjy94

Diff for: algorithm/Graph/topologicalSort/topologicalSort.js

@@ -0,0 +1,104 @@
+// lc207 Course Scheduel (https://leetcode.com/problems/course-schedule/)
+// BFS: compute indegrees
+/**
+ * @param {number} numCourses
+ * @param {number[][]} prerequisites
+ * @return {boolean}
+ */
+var canFinish = function (numCourses, prerequisites) {
+  const indegrees = [], preMatrix = [];
+  for (let [cur, pre] of prerequisites) {
+    if (!indegrees[cur]) {
+      indegrees[cur] = 0;
+    }
+    indegrees[cur]++;
+    if (!preMatrix[cur]) {
+      preMatrix[cur] = [];
+    }
+
+    preMatrix[cur][pre] = 1;
+  }
+
+  const queue = [];
+  for (let i = 0; i < numCourses; i++) {
+    if (!indegrees[i]) {
+      queue.push(i);
+    }
+  }
+
+  let count = 0;
+  while (queue.length) {
+    const cur = queue.shift();
+    count += 1;
+    for (let i = 0; i < numCourses; i++) {
+      if (preMatrix[i] && preMatrix[i][cur]) {
+        indegrees[i] -= 1;
+        if (indegrees[i] === 0) {
+          queue.push(i);
+        }
+      }
+    }
+  }
+
+  return count === numCourses;
+};
+
+// DFS
+const Node = function (val) {
+  this.val = val;
+  this.preRequists = [];
+  this.done = false;
+  this.todo = false;
+}
+
+const checkCycleExist = (cur, nodeStores) => {
+  if (cur.done) {
+    return false;
+  }
+
+  if (cur.todo) {
+    return true;
+  }
+
+  cur.todo = true;
+  for (let next of cur.preRequists) {
+    if (checkCycleExist(nodeStores.get(next), nodeStores)) {
+      return true;
+    }
+  }
+
+  cur.todo = false;
+  cur.done = true;
+  return false;
+};
+
+var canFinish = function (numCourses, prerequisites) {
+  const nodeStores = new Map();
+  if (prerequisites.length === 0) {
+    return true;
+  }
+
+  for (let [cur, pre] of prerequisites) {
+    if (!nodeStores.has(pre)) {
+      nodeStores.set(pre, new Node(pre));
+    }
+
+    if (!nodeStores.has(cur)) {
+      nodeStores.set(cur, new Node(cur));
+    }
+
+    nodeStores.get(cur).preRequists.push(pre);
+  }
+
+  for (let i = 0; i < numCourses; i++) {
+    if (!nodeStores.has(i)) {
+      continue;
+    }
+
+    if (checkCycleExist(nodeStores.get(i), nodeStores)) {
+      return false;
+    }
+  }
+
+  return true;
+};