update 207.course-schedule.java

Author: marklcrns

207.course-schedule.java

@@ -0,0 +1,112 @@
+import java.util.ArrayList;
+
+/*
+ * @lc app=leetcode id=207 lang=java
+ *
+ * [207] Course Schedule
+ *
+ * https://leetcode.com/problems/course-schedule/description/
+ *
+ * algorithms
+ * Medium (44.27%)
+ * Total Accepted:    536.4K
+ *
+ * There are a total of numCourses courses you have to take, labeled from 0 to
+ * numCourses - 1. You are given an array prerequisites where prerequisites[i]
+ * = [ai, bi] indicates that you must take course bi first if you want to take
+ * course ai.
+ *
+ *
+ * For example, the pair [0, 1], indicates that to take course 0 you have to
+ * first take course 1.
+ *
+ *
+ * Return true if you can finish all courses. Otherwise, return false.
+ *
+ *
+ * Example 1:
+ *
+ *
+ * Input: numCourses = 2, prerequisites = [[1,0]]
+ * Output: true
+ * Explanation: There are a total of 2 courses to take.
+ * To take course 1 you should have finished course 0. So it is possible.
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
+ * Output: false
+ * Explanation: There are a total of 2 courses to take.
+ * To take course 1 you should have finished course 0, and to take course 0 you
+ * should also have finished course 1. So it is impossible.
+ *
+ *
+ *
+ * Constraints:
+ *
+ *
+ * 1 <= numCourses <= 10^5
+ * 0 <= prerequisites.length <= 5000
+ * prerequisites[i].length == 2
+ * 0 <= ai, bi < numCourses
+ * All the pairs prerequisites[i] are unique.
+ *
+ *
+ */
+
+public class Solution {
+	public boolean canFinish(int numCourses, int[][] prerequisites) {
+		// Given infos
+		//
+		// prerequisites[] is the target class
+		// prerequisites[][] are the dependency classes
+
+		if (numCourses <= 1) {
+			return true;
+		}
+
+		ArrayList<Integer>[] graph = new ArrayList[numCourses];
+		// 0 = unvisited
+		// 1 = visited (but not all dependency nodes explored)
+		// 2 = completed (visited and all dependency nodes explored)
+		int[] visited = new int[numCourses];
+
+		for (int i = 0; i < graph.length; i++) {
+			graph[i] = new ArrayList<Integer>();
+		}
+
+		for (int[] pre : prerequisites) {
+			graph[pre[0]].add(pre[1]);
+		}
+
+		for (int i = 0; i < numCourses; ++i) {
+			if (!dfs(graph, i, visited)) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
+	public boolean dfs(ArrayList<Integer>[] graph, int course, int[] visited) {
+		// Cycle detected
+		if (visited[course] == 1) {
+			return false;
+		} else if (visited[course] == 2) {
+			return true;
+		}
+
+		visited[course] = 1;
+
+		for (int i = 0; i < graph[course].size(); ++i) {
+			if (!dfs(graph, graph[course].get(i), visited)) {
+				return false;
+			}
+		}
+
+		visited[course] = 2;
+		return true;
+	}
+}