here 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?
For example:
2, [[1,0]]
There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.
2, [[1,0],[0,1]]
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.
public class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
ArrayList<Set<Integer>> arrayList = new ArrayList<>();
for (int i = 0; i < numCourses; i++) {
arrayList.add(new HashSet<Integer>());
}
for (int i = 0; i < prerequisites.length; i++) {
arrayList.get(prerequisites[i][1]).add(prerequisites[i][0]);
}
int[] preNum = new int[numCourses];
for (int i = 0; i < numCourses; i++) {
Set<Integer> set = arrayList.get(i);
Iterator<Integer> iterator = set.iterator();
while (iterator.hasNext()) {
preNum[iterator.next()]++;
}
}
for (int i = 0; i < numCourses; i++) {
int j = 0;
for (; j < numCourses; j++) {
if (preNum[j] == 0) {
break;
}
}
if (j == numCourses) {
return false;
}
preNum[j] = -1;
Set<Integer> set = arrayList.get(j);
Iterator<Integer> iterator = set.iterator();
while (iterator.hasNext()) {
preNum[iterator.next()]--;
}
}
return true;
}
}