1462. Course Schedule IV

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DFS

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class Solution {
    public List<Boolean> checkIfPrerequisite(int numCourses, int[][] prerequisites, int[][] queries) {
        Map<Integer, List<Integer>> nodeToNodeListMap = new HashMap<>();
        for (int[] prerequisite : prerequisites) {
            int from = prerequisite[0], to = prerequisite[1];
            nodeToNodeListMap.computeIfAbsent(from, key -> new ArrayList<>()).add(to);
        }

        List<Boolean> resultList = new ArrayList<>(queries.length);
        Boolean[][] dfsCache = new Boolean[numCourses][numCourses];
        for (int[] query : queries) {
            resultList.add(dfs(dfsCache,nodeToNodeListMap, query[0], query[1]));
        }
        return resultList;
    }

    private Boolean dfs(Boolean[][] dfsCache, Map<Integer, List<Integer>> nodeToNodeListMap, int from, int to) {
        if (dfsCache[from][to] != null) {
            return dfsCache[from][to];
        }
        
        if (from == to) {
            return true;
        }

        for (int node : nodeToNodeListMap.getOrDefault(from, Collections.emptyList())) {
            if (dfs(dfsCache, nodeToNodeListMap, node, to)) {
                return true;
            }
        }

        dfsCache[from][to] = false;
        return false;
    }
}

BFS + Topological Sort

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class Solution {
    public List<Boolean> checkIfPrerequisite(int numCourses, int[][] prerequisites, int[][] queries) {
        int[] inDegrees = new int[numCourses];
        Map<Integer, List<Integer>> nodeToNodeListMap = new HashMap<>();
        for (int[] prerequisite : prerequisites) {
            int from = prerequisite[0], to = prerequisite[1];
            inDegrees[to]++;
            nodeToNodeListMap.computeIfAbsent(from, key -> new ArrayList<>()).add(to);
        }

        boolean[][] isPre = new boolean[numCourses][numCourses];
        Queue<Integer> zeroInDegreeCourseQueue = new LinkedList<>();
        for (int i = 0; i < inDegrees.length; i++) {
            if (inDegrees[i] == 0) {
                zeroInDegreeCourseQueue.add(i);
            }
        }

        while (!zeroInDegreeCourseQueue.isEmpty()) {
            int from = zeroInDegreeCourseQueue.poll();
            for (int to : nodeToNodeListMap.getOrDefault(from, Collections.emptyList())) {
                isPre[from][to] = true;
                // 枚举其他节点，尝试将其他节点与当前 to 节点标记连通
                for (int i = 0; i < numCourses; i++) {
                    isPre[i][to] |= isPre[i][from];
                }

                if (--inDegrees[to] == 0) {
                    zeroInDegreeCourseQueue.offer(to);
                }
            }
        }

        List<Boolean> resultList = new ArrayList<>(queries.length);
        for (int[] query : queries) {
            resultList.add(isPre[query[0]][query[1]]);
        }
        return resultList;
    }
}

References

1462. Course Schedule IV