递归return，循环调用的问题

/*

struct UndirectedGraphNode {

    int label;

    vector<struct UndirectedGraphNode *> neighbors;

    UndirectedGraphNode(int x) : label(x) {}

};*/

 

class Path {

public:

    bool checkPath(UndirectedGraphNode* a, UndirectedGraphNode* b) {

        return check(a, b) || check(b, a);

    }

    bool check(UndirectedGraphNode* a, UndirectedGraphNode* b) {

        if (a == NULL || b == NULL)

            return false;

        if (a == b)

            return true;

        map<UndirectedGraphNode*, bool> visited;

        visited[a] = true;

        for (int i = 0; i < a->neighbors.size(); i++)

        {

            if (a->neighbors[i] == b)

                return true;

            else

                if (visited[a->neighbors[i]] == false)

                    return checkPath(a->neighbors[i], b);

        }   

        return false;

    }

};