【Leetcode】Course Schedule #207

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.

Note:

The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how
a graph is represented.

一道拓扑排序题，不知道为什么Leetcode一些测试通不过，但是在本地测试那些cases都可以通过

enum Status {Unvisited = 0, Temp_Visited, Visited};

void constructGraph(int numCourses, vector<pair<int, int>> &prerequisites, unordered_map<int, vector<int>> &graph) {
unsigned long size = prerequisites.size();
for (int i = 0; i < size; ++i)
graph[prerequisites[i].first].push_back(prerequisites[i].second);
}

bool topo_sort_helper(int target, unordered_map<int, vector<int>> &graph, int *visit_record) {
if(visit_record[target] == Visited) return true;
else if (visit_record[target] == Temp_Visited) return false;
else {
visit_record[target] = Temp_Visited;
bool topo_success = false;
vector<int> connected_edge = graph[target];
unsigned long num_of_edges = connected_edge.size();
for (int i = 0; i < num_of_edges; ++i)
{
topo_success = topo_sort_helper(connected_edge[i], graph, visit_record);
if (topo_success == false) return false;
}
visit_record[target] = Visited;
return true;
}
}

bool topo_sort(int numCourses, unordered_map<int, vector<int>> &graph) {
int *visit_record = new int[numCourses];
memset(visit_record, Unvisited, numCourses);
for (int i = 0; i < numCourses; ++i)
{
if (visit_record[i] == Visited) continue;
else if (visit_record[i] == Unvisited) {
if (topo_sort_helper(i, graph, visit_record) == false)
{
delete visit_record;
return false;
}
}
}
delete visit_record;
return true;
}

bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
unordered_map<int, vector<int>> graph;
constructGraph(numCourses, prerequisites, graph);
return topo_sort(numCourses, graph);
}