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CourseSchedule.java
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98 lines (82 loc) · 3.25 KB
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/*
There are a total of n courses you have to take, labeled from 0 to n-1.
Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]
Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?
Example 1:
Input: 2, [[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: 2, [[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.
Note:
The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
You may assume that there are no duplicate edges in the input prerequisites
*/
import java.util.*;
public class CourseSchedule {
/*
Topological sort can be started with the first node OR the last node.
The only difference in the process is removing the out-come edge OR in-come edge.
In this case, I choose start with the first node(indegree == 0);
*/
// more efficient
public boolean canFinish0(int numCourses, int[][] prerequisites) {
int[] inDegree = new int[numCourses];
for(int i = 0; i <= prerequisites.length - 1; i++) {
inDegree[prerequisites[i][0]]++;
}
Stack<Integer> stack = new Stack<>();
for(int i = 0; i <= inDegree.length - 1; i++) {
if(inDegree[i] == 0) {
stack.push(i); // !!
}
}
int zeroDegreeCount = stack.size();
while(!stack.isEmpty()) {
int zeroInDegreeNode = stack.pop();
for(int i = 0; i <= prerequisites.length - 1; i++) {
if(zeroInDegreeNode == prerequisites[i][1]) {
inDegree[prerequisites[i][0]]--;
if(inDegree[prerequisites[i][0]] == 0) {
stack.push(prerequisites[i][0]);
zeroDegreeCount++;
}
}
}
}
return (zeroDegreeCount == numCourses);
}
public static boolean canFinish(int numCourses, int[][] prerequisites) {
if (numCourses <= 0)
return false;
Queue<Integer> queue = new LinkedList<>();
int[] inDegree = new int[numCourses];
for (int i = 0; i < prerequisites.length; i++) { // set indgree for each node
inDegree[prerequisites[i][0]]++;
}
for (int i = 0; i < inDegree.length; i++) { // add all zero indegree nodes onto queue
if (inDegree[i] == 0)
queue.offer(i);
}
while (!queue.isEmpty()) {
int zeroInDgreeNode = queue.poll();
for (int i = 0; i < prerequisites.length; i++) {
if (zeroInDgreeNode == prerequisites[i][1]) {
inDegree[prerequisites[i][0]]--;
if (inDegree[prerequisites[i][0]] == 0)
queue.offer(prerequisites[i][0]);
}
}
}
for (int i = 0; i < inDegree.length; i++) {
if (inDegree[i] != 0)
return false;
}
return true;
}
}