有向图的拓扑排序

只有有向图才有的拓扑排序

代码

class Solution {
public:
    //注意，邻接链表存储的是该节点可以到达的节点
    //保存的是邻接链表
    vector<vector<int>> graph;
    vector<int> toposort(vector<int>& nums, int numCourses) {
        vector<int> ans;
        queue<int> q;
        //第一步，将入度表中值为0的节点进队
        for(int i = 0; i < numCourses; i++) {
            if(nums[i] == 0) {
                q.push(i);
                ans.push_back(i);
            }
        }
        while(!q.empty()) {
            int front = q.front();
            q.pop();
            for(auto i : graph[front]) {
                nums[i]--;
                if(nums[i] == 0) {
                    q.push(i);
                    ans.push_back(i);
                }
            }
        }
 
        return ans;
    }
    vector<int> findOrder(int numCourses, vector<vector<int>>& prerequisites) {
        graph.resize(numCourses + 1);
        for(auto it : prerequisites) {
            graph[it[1]].push_back(it[0]);
        }
        //保存的是入度表
        vector<int> nums(numCourses, 0);
        //初始化入度表
        for(auto it : prerequisites) {
            nums[it[0]]++;
        }
        vector<int> ans = toposort(nums, numCourses);
        if(ans.size() == numCourses) {
            return ans;
        }
        return vector<int>();

    }
};