实质上就是图的遍历,先要建图,然后对图进行深度优先的遍历,把dfs函数当成游离在图里的指针。
class Solution {
public:
vector<bool> visited;
vector<bool> onpath;
bool hasCycle = false;
vector<vector<int>> buildGraph(int numCourses, vector<vector<int>>& prerequisites){
vector<vector<int>> edges(numCourses);
for(auto pair: prerequisites){
edges[pair[1]].emplace_back(pair[0]);
}
return edges;
}
void dfs(vector<vector<int>>& edges, int s){
if(onpath[s]){
hasCycle = true;
}
if(visited[s]||hasCycle) return;
visited[s] = true;
onpath[s] = true;
for(int x: edges[s]){
dfs(edges, x);
}
onpath[s] = false;
}
bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
visited.resize(numCourses);
onpath.resize(numCourses);
vector<vector<int>> edges = buildGraph(numCourses, prerequisites);
for(int i=0; i<numCourses; ++i){
dfs(edges, i);
}
return !hasCycle;
}
};