Course Schedule

Description

https://leetcode.com/problems/course-schedule/description/

Stupid Solution

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        unordered_map<int, unordered_set<int> > adj;
        for (auto& pa : prerequisites) {
            adj[pa.first].insert(pa.second);
        }
        for (int i = 0; i < numCourses; ++i)
            if (adj.find(i) == adj.end())
                for (auto iter = adj.begin(); iter != adj.end(); ++iter)
                    iter->second.erase(i);
        
        while(!adj.empty()) {
            bool flag = false;
            auto iter = adj.begin();
            while(iter != adj.end()) {
                auto cur = iter;
                ++iter;
                if (cur->second.empty()) {
                    flag = true;
                    int key = cur->first;
                    adj.erase(cur);
                    for (auto it = adj.begin(); it != adj.end(); ++it)
                        it->second.erase(key);
                }
            }
            if (!flag && !adj.empty()) return false;
        }
        return true;
    }
};

Reasonable Solution

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        vector<int> degree(numCourses, 0);
        vector<vector<int>> graph(numCourses, vector<int>() );
        for (auto& pa : prerequisites) {
            degree[pa.first] += 1;
            graph[pa.second].push_back(pa.first);
        }
        stack<int> waitingList;
        for (int i = 0; i < numCourses; ++i) {
            if (degree[i] == 0){
                waitingList.push(i);
            }
        }
        while (!waitingList.empty()) {
            int item = waitingList.top();
            waitingList.pop();
            vector<int>& lis = graph[item];
            for (int item : lis) {
                --degree[item];
                if (degree[item] == 0) {
                    waitingList.push(item);
                }
            }
        }
        
        for (int i = 0; i < numCourses; ++i)
            if (degree[i] != 0) return false;
        
        return true;
    }
};