[CareerCup] 4.2 Route between Two Nodes in Directed Graph 有向图中两点的路径

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

LeetCode和CareerCup中关于图的题都不是很多，LeetCode中只有三道，分别是Clone Graph 无向图的复制，Course Schedule 课程清单 和 Course Schedule II 课程清单之二。目前看来CareerCup中有关图的题在第四章中仅此一道，这是一道关于有向图的题，书中是用java来做的，我们用c++来做时要定义图和节点，这里参考了之前那道Clone Graph 无向图的复制中关于无向图的定义，并且在里面加入了一个枚举类变量state，来帮助我们遍历。这种找两点之间路径的题就是遍历的问题，可以用BFS或DFS来解，先来看BFS的解法，如下所示：

//Definition for directed graph.

enum State {

Unvisited, Visited, Visiting

};

struct DirectedGraphNode {

int label;

State state;

vector neighbors;

DirectedGraphNode(int x) : label(x) {};

};

struct DirectedGraph {

vector nodes;

};

class Solution {

public:

bool search(DirectedGraph *g, DirectedGraphNode *start, DirectedGraphNode *end) {

queue 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;

}

};

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