图中两个点之间的路线

给出一张有向图，设计一个算法判断两个点 s 与 t 之间是否存在路线。

您在真实的面试中是否遇到过这个题？ 

Yes

样例

如下图:

A----->B----->C
 \     |
  \    |
   \   |
    \  v
     ->D----->E

for s = B and t = E, return true

for s = D and t = C, return false

/**
 * Definition for Directed graph.
 * struct DirectedGraphNode {
 *     int label;
 *     vector<DirectedGraphNode *> neighbors;
 *     DirectedGraphNode(int x) : label(x) {};
 * };
 */
class Solution {
public:
    /**
     * @param graph: A list of Directed graph node
     * @param s: the starting Directed graph node
     * @param t: the terminal Directed graph node
     * @return: a boolean value
     */
    bool hasRoute(vector<DirectedGraphNode*> graph,
                  DirectedGraphNode* s, DirectedGraphNode* t) {
        // write your code here
        vector<DirectedGraphNode*> buf;
	    bool result = false;
	    visit(s, t, buf, result);

	    return result;
    }
private:
    void visit(DirectedGraphNode *s, DirectedGraphNode *t, 
			vector<DirectedGraphNode*> &buf, bool &result)
    {
	    if (result)
	    {
		    return;
	    }

	    if (find(buf.begin(), buf.end(), s) != buf.end())
	    {
		    return;
	    }

	    if (s == t)
	    {
		    result = true;
		    return;
	    }

	    buf.push_back(s);
	    for (int i = 0; i < s->neighbors.size(); i++)
	    {
		    visit(s->neighbors[i], t, buf, result);
		    if (result)
		    {
			    break;
		    }
	    }
    }
};