176.Route Between Two Nodes in Graph-图中两个点之间的路线（中等题）

图中两个点之间的路线

1. 题目

   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。

2. 样例

   如下图:
   
   for s = B and t = E, return true
   for s = D and t = C, return false

3. 题解

   直接使用BFS就可以了，如从起始节点开始，应依次先访问这个节点所指向的其他节点，也就是neigbors中存储的节点，再继续访问neigbors节点的neigbors，数据结构选用队列。

/**
 * Definition for Directed graph.
 * class DirectedGraphNode {
 *     int label;
 *     ArrayList<DirectedGraphNode> neighbors;
 *     DirectedGraphNode(int x) {
 *         label = x;
 *         neighbors = new ArrayList<DirectedGraphNode>();
 *     }
 * };
 */
public class Solution {
   /**
     * @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
     */
    public boolean hasRoute(ArrayList<DirectedGraphNode> graph, 
                            DirectedGraphNode s, DirectedGraphNode t) {
        if (s == t)
        {
            return true;
        }

        HashSet<DirectedGraphNode> visited = new HashSet<DirectedGraphNode>();
        Queue<DirectedGraphNode> queue = new LinkedList<DirectedGraphNode>();

        queue.offer(s);
        visited.add(s);
        while (!queue.isEmpty()) 
        {
            DirectedGraphNode node = queue.poll();
            for (int i = 0; i < node.neighbors.size(); i++) 
            {
                if (visited.contains(node.neighbors.get(i))) 
                {
                    continue;
                }
                visited.add(node.neighbors.get(i));
                queue.offer(node.neighbors.get(i));
                if (node.neighbors.get(i) == t) 
                {
                    return true;
                }
            }
        }

        return false;
    }
}

Last Update 2016.10.25