Route Between Two Nodes in Graph

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

Have you met this question in a real interview?

Yes

Example

Given graph:

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

for s = B and t = E, return true

for s = D and t = C, return false

Analysis:

Set + DFS

 /**
  * 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) {
         Set<DirectedGraphNode> visited = new HashSet<DirectedGraphNode>();
         return dfs(graph, s, t, visited);
     }

     public boolean dfs(ArrayList<DirectedGraphNode> graph,
                        DirectedGraphNode s, DirectedGraphNode t,
                        Set<DirectedGraphNode> visited) {
         // corner cases
         if (s == null || t == null) return false;
         if (s == t) {
             return true;
         } else {
             // flag visited node, avoid cylic
             visited.add(s);
             // compare unvisited neighbor nodes recursively
             if (s.neighbors.size() > ) {
                 for (DirectedGraphNode node : s.neighbors) {
                     if (visited.contains(node)) continue;
                     if (dfs(graph, node, t, visited)) return true;
                 }
             }
         }

         return false;
     }
 }