sdev333 final exam study guide graph traversal DFS style

src/Practice.java

@@ -1,6 +1,9 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashSet;
 
 public class Practice {
 
@@ -25,7 +28,31 @@ public class Practice {
    * @return the number of vertices with odd values reachable from the starting vertex
    */
   public static int oddVertices(Vertex<Integer> starting) {
-    return 0;
+    if (starting == null) return 0;
+
+    //initialize visited Set
+    Set<Vertex<Integer>> visited = new HashSet<>();
+
+    //start recursive traversal with starting vertex
+    return oddVerticesHelper(starting, visited);
+  }
+  //helper method-
+  //no extra shared mutable variables needed—recursion handles
+  //accumulation through returns
+  private static int oddVerticesHelper(Vertex<Integer> current, Set<Vertex<Integer>> visited) {
+    //base case-if we've already visited this vertex return immediately
+    if(visited.contains(current)) return 0;
+    //mark current as visited to avoid infinite loops
+    visited.add(current);
+    //count this vertex.data if odd
+    int count = (current.data % 2 != 0) ? 1 : 0;
+    //for-each iterates each neighbor of current
+    for (Vertex<Integer> neighbor : current.neighbors) {
+      //recursion step adds up counts returned from neighbors
+      count += oddVerticesHelper(neighbor, visited);
+    }
+    //return total count accumulated from this vertex downward
+    return count;
   }
 
   /**
@@ -48,7 +75,28 @@ public static int oddVertices(Vertex<Integer> starting) {
    */
   public static List<Integer> sortedReachable(Vertex<Integer> starting) {
     // Unimplemented: perform a depth-first search and sort the collected values.
-    return null;
+    //base case
+    if (starting == null) return new ArrayList<>();
+    //initialize empty visited Set/empty results List
+    Set<Vertex<Integer>> visited = new HashSet<>();
+    List<Integer> results = new ArrayList<>();
+    //
+    sortedReachableHelper(starting, visited, results);
+    //sort results List into asc order
+    Collections.sort(results);
+    return results;
+  }
+  private static void sortedReachableHelper(Vertex<Integer> current, Set<Vertex<Integer>> visited, List<Integer> results) {
+    //base case
+    if (visited.contains(current)) return;
+    //add current and current.data to visited and results trackers
+    visited.add(current);
+    results.add(current.data);
+    //for each to iterate through each neighbor of current
+    for (Vertex<Integer> neighbor : current.neighbors) {
+      //recurse all neighbors
+      sortedReachableHelper(neighbor, visited, results);
+    }
   }
 
   /**
@@ -57,14 +105,37 @@ public static List<Integer> sortedReachable(Vertex<Integer> starting) {
    * It is assumed that there are no duplicate vertices.
    * If the starting vertex is not present as a key in the map, returns an empty list.
    *
-   * @param graph a map representing the graph
+   * @param graph a map representing the adjacency list of the graph
    * @param starting the starting vertex value
    * @return a sorted list of all reachable vertex values
    */
   public static List<Integer> sortedReachable(Map<Integer, Set<Integer>> graph, int starting) {
-    return null;
+    //base case (  more generic: return Collections.emptySet();  )
+    if (graph == null || !graph.containsKey(starting)) return new ArrayList<>();
+    //instantiate visited Set
+    Set<Integer> visited = new HashSet<>();
+
+    sortedReachableHelper(graph, starting, visited);
+
+    //convert visited data into List format
+    List<Integer> sorted = new ArrayList<>(visited);
+    //sort newly formed List
+    Collections.sort(sorted);
+    //return List of sorted reachable 'vertices'
+    return sorted;
   }
 
+  private static void sortedReachableHelper(Map<Integer, Set<Integer>> graph, int current, Set<Integer> visited) {
+    //base case - if we've been here already then return immediately
+    if (visited.contains(current)) return;
+    //add current to visited
+    visited.add(current);
+    //for each avoids getting a null pointer exception or needing 
+    //an extra check for null values explicitly
+    for (Integer neighbor : graph.getOrDefault(current, Collections.emptySet())) { //recursively transverse to fill up visited Set
+      sortedReachableHelper(graph, neighbor, visited);
+    }
+  }
   /**
    * Returns true if and only if it is possible both to reach v2 from v1 and to reach v1 from v2.
    * A vertex is always considered reachable from itself.
@@ -80,6 +151,29 @@ public static List<Integer> sortedReachable(Map<Integer, Set<Integer>> graph, in
    * @return true if there is a two-way connection between v1 and v2, false otherwise
    */
   public static <T> boolean twoWay(Vertex<T> v1, Vertex<T> v2) {
+    //base case
+    if (v1 == null || v2 == null) return false;
+    //instantiate Set inside helper method
+    //call helper method
+    boolean oneWay = twoWayHelper(v1, v2, new HashSet<>());
+    //repeat for opposite direction
+
+    //call helper method
+    boolean otherWay = twoWayHelper(v2, v1, new HashSet<>());
+    return oneWay && otherWay;
+  }
+  private static <T> boolean twoWayHelper(Vertex<T> current, Vertex<T> v2, Set<Vertex<T>> visited) {
+    //base case -> successfully reached target vertex
+    if (current == v2) return true;
+    //already visited? stop recursion!
+    if (visited.contains(current)) return false;
+    //add current to visited to avoid looping
+    visited.add(current);
+    //for each iterate all neighbors of current
+    for (Vertex<T> neighbor : current.neighbors) {
+      //recursively transverse each neighbor of current
+      if (twoWayHelper(neighbor, v2, visited)) return true;
+    }
     return false;
   }
 
@@ -96,6 +190,22 @@ public static <T> boolean twoWay(Vertex<T> v1, Vertex<T> v2) {
    * @return whether there exists a valid positive path from starting to ending
    */
   public static boolean positivePathExists(Map<Integer, Set<Integer>> graph, int starting, int ending) {
+    //base case
+    if (starting < 0 || ending < 0 || graph == null || !graph.containsKey(starting) || !graph.containsKey(ending)) return false;
+    //call helper method inside boolean solution return
+    return positivePathExistsHelper(graph, starting, ending, new HashSet<>());
+  }
+  private static boolean positivePathExistsHelper(Map<Integer, Set<Integer>> graph, int current, int ending, Set<Integer> visited) {
+    //base case -> have we reached ending via current?
+    if (current == ending) return true;
+    //have we been here already? if so return false
+    if (visited.contains(current)) return false;
+    //add current to visited as tracking to avoid looping
+    visited.add(current);
+    //for each to iterate neighbors of current
+    for (Integer neighbor : graph.getOrDefault(current, Collections.emptySet())) {
+      if (neighbor > 0 && positivePathExistsHelper(graph, neighbor, ending, visited)) return true;
+    }
     return false;
   }
 
@@ -109,6 +219,27 @@ public static boolean positivePathExists(Map<Integer, Set<Integer>> graph, int s
    * @return true if a person in the extended network works at the specified company, false otherwise
    */
   public static boolean hasExtendedConnectionAtCompany(Professional person, String companyName) {
+    //base case
+    if (person == null) return false;
+    //call helper
+    return hasExtendedConnectionAtCompanyHelper(person, companyName, new HashSet<>());
+  }
+  private static boolean hasExtendedConnectionAtCompanyHelper(Professional currentPerson, String companyName, Set<Professional> visited) {
+    //base case: if we've reached via DFS a person
+    //working at same company as starting person
+    //return true; **use .equals() for String comparisons
+    if (currentPerson.getCompany().equals(companyName)) return true;
+    //have we visited this person already? return false!
+    if (visited.contains(currentPerson)) return false;
+    //add currentPerson to visited Set
+    visited.add(currentPerson);
+    //for-each iterates all connections currentPerson has
+    for (Professional connection : currentPerson.getConnections()) {
+      //if recursion step is true, return true
+      if (hasExtendedConnectionAtCompanyHelper(connection, companyName, visited)) {
+        return true;
+      } 
+    }
     return false;
   }
 }