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<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;
}
};