Course Schedule

2015.5.8 01:26

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:

This problem is about topological sort. Watch out for multigraph.

Accepted code:

# My code looks ugly enough...
class Solution:
# @param {integer} numCourses
# @param {integer[][]} prerequisites
# @return {boolean}
def canFinish(self, numCourses, prerequisites):
e = prerequisites
n = numCourses

g = [set() for i in xrange(n)]
ind = [0 for i in xrange(n)]
ec = len(e)
for i in xrange(ec):
ec = 0
for i in xrange(n):
for j in g[i]:
ind[j] += 1
ec += len(g[i])
b = [False for i in xrange(n)]
while True:
i = 0
while i < n:
if ind[i] == 0 and not b[i]:
break
i += 1
if i == n:
break
for j in g[i]:
ind[j] -= 1
g[i] = set()
b[i] = True
i = 0
while i < n:
if not b[i]:
return False
i += 1
return True

posted on 2015-05-08 01:34  zhuli19901106  阅读(376)  评论(0编辑  收藏  举报