Course Schedule
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?
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.
Solution
class Solution(object):
def canFinish(self, numCourses, prerequisites):
"""
:type numCourses: int
:type prerequisites: List[List[int]]
:rtype: bool
"""
graph = [[] for _ in xrange(numCourses)]
visit = [0 for _ in xrange(numCourses)]
for x,y in prerequisites:
graph[x].append(y)
def dfs(i):
if visit[i] == -1:
return False
if visit[i] == 1:
return True
visit[i] = -1
for j in graph[i]:
if not dfs(j):
return False
visit[i] = 1
return True
for i in xrange(numCourses):
if not dfs(i):
return False
return True
public class Solution {
public boolean canFinish(int numCourses, int[][] prerequisites) {
int [][] matrix = new int[numCourses][numCourses];
int [] inDegree = new int[numCourses];
for(int [] p : prerequisites) {
int course = p[0], pre = p[1];
if(matrix[pre][course] == 0)
inDegree[course] ++;
matrix[pre][course] = 1;
}
int count = 0;
Queue<Integer> q = new LinkedList();
for(int i = 0;i< inDegree.length;i++)
if(inDegree[i] == 0) q.offer(i);
while(!q.isEmpty()) {
int cur = q.poll();
count += 1;
for(int i = 0;i<numCourses;i++) {
if(matrix[cur][i] != 0) {
if(--inDegree[i] == 0)
q.offer(i);
}
}
}
return count == numCourses;
}
}