207 Course Schedule

參考了geeksforgeeks 有向圖尋找環。代碼如下：

public class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        if (prerequisites == null || prerequisites.length == ) {
            return true;
        }

        Map<Integer, List<Integer>> map = new HashMap<Integer, List<Integer>>();
        for (int i = ; i < prerequisites.length; i++) {
            int[] pair = prerequisites[i];
            int from = pair[];
            int to = pair[];
            if (map.containsKey(from)) {
                map.get(from).add(to);
            } else {
                List<Integer> list = new ArrayList<Integer>();
                list.add(to);
                map.put(from, list); // map 0, <1> || 1, empty
            }

            if (!map.containsKey(to)) {
                map.put(to, new ArrayList<Integer>());
            }
        }

        boolean[] visited = new boolean[numCourses];
        boolean[] recStack = new boolean[numCourses];

        for (int i : map.keySet()) {
            if (isCyclic(i, visited, recStack, map)) {
                return false;
            }
        }
        return true;
    }

    private boolean isCyclic(int v, boolean[] visited, boolean[] recStack, Map<Integer, List<Integer>> map) {
        if (!visited[v]) {
            visited[v] = true;
            recStack[v] = true;
            List<Integer> adj = map.get(v);
            for (int i = ; i < adj.size(); i++) {
                int num = adj.get(i);
                if (!visited[num] && isCyclic(num, visited, recStack, map)) {
                    return true;
                } else if (recStack[num]) {
                    return true;
                }
            }
        }
        recStack[v] = false;
        return false;
    }
}