leetcode之Course Schedule

题目：

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.

解答：

题目很明显是一个图，利用拓扑排序可以确定是不是能够进行完成上课

class Solution {
public:
bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
vector<int> indegree(numCourses,0);
int size = prerequisites.size();
if(size == 1 || size == 0)
return true;
map<int, vector<int>>m;
for(int i = 0;i < size;++i)
{
indegree[prerequisites[i].first]++;
m[prerequisites[i].second].push_back(prerequisites[i].first);
}
int cnt = 0;
queue<int> q;
for(int i = 0;i < numCourses;++i)
{
if(indegree[i] == 0)
q.push(i);
}
while(!q.empty())
{
int s = q.size();
while(s--)
{
int tmp = q.front();
q.pop();
cnt++;
for(int j = 0;j < m[tmp].size(); ++j)
{
indegree[m[tmp][j]]--;
}
indegree[tmp]--;
}
for(int i = 0;i < numCourses; ++i)
{
if(indegree[i] == 0)
q.push(i);
}
}
return numCourses == cnt ? true :false;
}
};