4.2 Given a directed graph, design an algorithm to find out whether there is a route between two nodes.

LeetCode和CareerCup中關於圖的題都不是很多，LeetCode中只有三道，分別是Clone Graph 無向圖的複製，Course Schedule 課程清單 和 Course Schedule II 課程清單之二。目前看來CareerCup中有關圖的題在第四章中僅此一道，這是一道關於有向圖的題，書中是用java來做的，我們用c++來做時要定義圖和節點，這裡參考了之前那道

//Definition for directed graph.
enum State {
    Unvisited, Visited, Visiting
};

struct DirectedGraphNode {
   int label;
   State state;
   vector<DirectedGraphNode *> neighbors;
   DirectedGraphNode(
 
int x) : label(x) {};
};

struct DirectedGraph {
    vector<DirectedGraphNode*> nodes;
};

class Solution {
public:
    bool search(DirectedGraph *g, DirectedGraphNode *start, DirectedGraphNode *end) {
        queue<DirectedGraphNode*> q;
        for (auto a : g->nodes) a->state = Unvisited;
        start
 
->state = Visiting;
        q.push(start);
        while (!q.empty()) {
            DirectedGraphNode *node = q.front(); q.pop();
            for (auto a : node->neighbors) {
                if (a->state == Unvisited) {
                    if (a == end) return true;
                    else {
                        a->state = Visiting;
                        q.push(a);
                    }
                }
            }    
            node->state = Visited;
        }
        return false;
    }
};