diff --git a/src/Practice.java b/src/Practice.java
index 01da8d0..f6b9d95 100644
--- a/src/Practice.java
+++ b/src/Practice.java
@@ -1,153 +1,254 @@
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
 public class Practice {
-    /**
-     * Returns the sum of the odd numbers in the array.
-     * 
-     * Returns 0 if the array is null or has no odd numbers.
-     * 
-     * @param nums an array of numbers
-     * @return the sum of the odd numbers in the array
-     */
-    public static int oddSum(int[] nums) {
-        return 0;
+  /**
+   * Returns the sum of the odd numbers in the array.
+   * 
+   * Returns 0 if the array is null or has no odd numbers.
+   * 
+   * @param nums an array of numbers
+   * @return the sum of the odd numbers in the array
+   */
+  public static int oddSum(int[] nums) {
+    if(nums == null) return 0;
+    int sum = 0;
+    for(int num : nums) {
+      if(num % 2 != 0) {
+        sum += num;
+      }
     }
+    return sum;
+  }
 
-    /**
-     * Returns the shortest word in the Set.
-     * 
-     * If multiple words are tied for shortest, returns the one that is smallest
-     * lexicographically.
-     * 
-     * @param words a set of words
-     * @return the shortest word in the set with a lexicographic tiebreaker
-     * @throws IllegalArgumentException if words is empty
-     * @throws NullPointerException if words is null
-     */
-    public static String shortestWord(Set<String> words) {
-        return null;
+  /**
+   * Returns the shortest word in the Set.
+   * 
+   * If multiple words are tied for shortest, returns the one that is smallest
+   * lexicographically.
+   * 
+   * @param words a set of words
+   * @return the shortest word in the set with a lexicographic tiebreaker
+   * @throws IllegalArgumentException if words is empty
+   * @throws NullPointerException if words is null
+   */
+  public static String shortestWord(Set<String> words) {
+    if(words == null) throw new NullPointerException();
+    if (words.size() == 0) throw new IllegalArgumentException();
+    List<String> wordList = new ArrayList<>();
+    wordList.addAll(words);
+    Collections.sort(wordList);
+    String shortest = wordList.get(0);
+    for( String word: wordList) {
+      if(word.length() < shortest.length()) {
+        shortest = word;
+      }
     }
+    return shortest;
+  }
 
-    /**
-     * Returns a set of all the names of people that are 18 years of age or older.
-     * 
-     * The input maps name to age in years.
-     * 
-     * @param ages mapping of name to age
-     * @return the set of all names of people >= 18 years old
-     * @throws NullPointerException if ages is null
-     */
-    public static Set<String> adults(Map<String, Integer> ages) {
-        return null;
+  /**
+   * Returns a set of all the names of people that are 18 years of age or older.
+   * 
+   * The input maps name to age in years.
+   * 
+   * @param ages mapping of name to age
+   * @return the set of all names of people >= 18 years old
+   * @throws NullPointerException if ages is null
+   */
+  public static Set<String> adults(Map<String, Integer> ages) {
+    if(ages == null) throw new NullPointerException();
+    Set<String> nameSet = new HashSet<>();
+    for (String key : ages.keySet()) {
+      if(ages.get(key) >= 18) {
+        nameSet.add(key);
+      }
     }
+    return nameSet;
+  }
 
-    /**
-     * Returns the biggest number in a linked list.
-     * 
-     * @param head the head of the linked list
-     * @return the biggest number in the list
-     * @throws IllegalArgumentException if head is null
-     */
-    public static int biggestNumber(ListNode<Integer> head) {
-        return 0;
+  /**
+   * Returns the biggest number in a linked list.
+   * 
+   * @param head the head of the linked list
+   * @return the biggest number in the list
+   * @throws IllegalArgumentException if head is null
+   */
+  public static int biggestNumber(ListNode<Integer> head) {
+    if(head == null) throw new IllegalArgumentException();
+    ListNode<Integer> current = head;
+    int biggest = current.data;
+    while(current != null) {
+      if(current.data > biggest) {
+        biggest = current.data;
+      }
+      current = current.next;
     }
+    return biggest;
+  }
 
-    /**
-     * Returns a frequency map counting how frequently items appear in a linked list.
-     * 
-     * Example:
-     *   Input: a -> x -> a -> a -> x -> y
-     *   Output: {a:3, x:2, y: 1}
-     * 
-     * Returns an empty map if head is null
-     * 
-     * @param <T> the type of data held by the list
-     * @param head the head of the list
-     * @return a frequency map of values in the list
-     */
-    public static <T> Map<T, Integer> frequencies(ListNode<T> head) {
-        return null;
+  /**
+   * Returns a frequency map counting how frequently items appear in a linked list.
+   * 
+   * Example:
+   *   Input: a -> x -> a -> a -> x -> y
+   *   Output: {a:3, x:2, y: 1}
+   * 
+   * Returns an empty map if head is null
+   * 
+   * @param <T> the type of data held by the list
+   * @param head the head of the list
+   * @return a frequency map of values in the list
+   */
+  public static <T> Map<T, Integer> frequencies(ListNode<T> head) {
+    Map<T, Integer> map = new HashMap<>();
+    ListNode<T> current = head;
+    while(current != null) {
+      T key = current.data;
+      if(!map.containsKey(key)) {
+        map.put(key, 1);
+      }
+      else {
+        map.put(key, map.get(key) +1);
+      }
+      current = current.next;
     }
+    return map;
+  }
 
+  /**
+   * Returns the number of levels in the tree.
+   * 
+   * An empty tree has 0 levels, a tree with only a root has 1 level.
+   * 
+   * @param root the root of the tree
+   * @return the number of levels in the tree
+   */
+  public static int levelCount(BinaryTreeNode<?> root) {
+    if(root == null) return 0;
+    int left = levelCount(root.left);
+    int right = levelCount(root.right);
+    int levels = Math.max(left, right);
+    return levels + 1;
+  }
 
-    /**
-     * Returns the number of levels in the tree.
-     * 
-     * An empty tree has 0 levels, a tree with only a root has 1 level.
-     * 
-     * @param root the root of the tree
-     * @return the number of levels in the tree
-     */
-    public static int levelCount(BinaryTreeNode<?> root) {
-        return 0;
-    }
+  /**
+   * Returns the sum at a specified level in a binary tree.
+   * 
+   * For example, if the given level was 3:
+   *       5
+   *     /   \
+   *    8     4
+   *   / \   / 
+   *  7  9  2
+   *    /
+   *   1
+   * 
+   * Nodes at level 3: 7, 9, and 2
+   * Sum of nodes at level 3: 18 
+   * 
+   * The root is considered to be at level 1.
+   * 
+   * Returns 0 if the tree is empty or if the level is not present in the tree.
+   * 
+   * @param root the root of the binary tree
+   * @param level the level to sum
+   * @return the sum of the nodes at the given level
+   */
+  public static int sumAtLevel(BinaryTreeNode<Integer> root, int level) {
+    if(root == null) return 0;
+    if (level == 1) return root.data;
+    int left = sumAtLevel(root.left, level -1);
+    int right = sumAtLevel(root.right, level -1);
+    return left + right;
+  }
+
+  /**
+   * Returns true if the sum of the values in a given tree is equal to the sum
+   * of the values in the given list. 
+   * 
+   * An empty tree or list is considered to have a sum of 0.
+   * 
+   * @param root The root of the binary tree
+   * @param head The head of the linked list
+   * @return true if the sums are equal, false otherwise
+   */
+  public static boolean sumMatch(BinaryTreeNode<Integer> root, ListNode<Integer> head) {
+    if (sumBinaryTree(root) == sumLinkedList(head)) return true;
+    return false;
+  }
 
+  public static int sumBinaryTree(BinaryTreeNode<Integer> root) {
+    if(root == null) return 0;
+    return sumBinaryTree(root.left) + sumBinaryTree(root.right) + root.data;
+  }
 
-    /**
-     * Returns the sum at a specified level in a binary tree.
-     * 
-     * For example, if the given level was 3:
-     *       5
-     *     /   \
-     *    8     4
-     *   / \   / 
-     *  7  9  2
-     *    /
-     *   1
-     * 
-     * Nodes at level 3: 7, 9, and 2
-     * Sum of nodes at level 3: 18 
-     * 
-     * The root is considered to be at level 1.
-     * 
-     * Returns 0 if the tree is empty or if the level is not present in the tree.
-     * 
-     * @param root the root of the binary tree
-     * @param level the level to sum
-     * @return the sum of the nodes at the given level
-     */
-    public static int sumAtLevel(BinaryTreeNode<Integer> root, int level) {
-        return 0;
+  public static int sumLinkedList(ListNode<Integer> head) {
+    if(head == null) return 0;
+    ListNode<Integer> current = head;
+    int sum = 0;
+    while(current != null) {
+      sum += current.data;
+      current = current.next;
     }
+    return sum;
+  }
 
+  /**
+   * Returns the sum of all the vertices in a graph that are reachable from a given
+   * starting vertex.
+   * 
+   * Returns 0 if the starting vertex is null.
+   * 
+   * @param start the starting vertex
+   * @return the sum of all the vertices
+   */
+  public static int graphSum(Vertex<Integer> start) {
+    Set<Vertex<Integer>> visited = new HashSet<>();
+    return graphSum(start, visited); 
+  }
 
-    /**
-     * Returns true if the sum of the values in a given tree is equal to the sum
-     * of the values in the given list. 
-     * 
-     * An empty tree or list is considered to have a sum of 0.
-     * 
-     * @param root The root of the binary tree
-     * @param head The head of the linked list
-     * @return true if the sums are equal, false otherwise
-     */
-    public static boolean sumMatch(BinaryTreeNode<Integer> root, ListNode<Integer> head) {
-        return false;
+  public static int graphSum(Vertex<Integer> start, Set<Vertex<Integer>> visited) {
+    if(start == null || visited.contains(start)) return 0;
+    visited.add(start);
+    int sum = 0;
+    sum += start.data;
+    for(Vertex<Integer>neighbor : start.neighbors) {
+      int neighborSum = graphSum(neighbor, visited);
+      sum += neighborSum;
     }
+    return sum;
+  }
 
-    /**
-     * Returns the sum of all the vertices in a graph that are reachable from a given
-     * starting vertex.
-     * 
-     * Returns 0 if the starting vertex is null.
-     * 
-     * @param start the starting vertex
-     * @return the sum of all the vertices
-     */
-    public static int graphSum(Vertex<Integer> start) {
-        return 0;
-    }
+  /**
+   * Returns the count of vertices in a graph that have an outdegree of 0.
+   * 
+   * Returns 0 if the starting vertex is null.
+   * 
+   * @param start the entrypoint to the graph
+   * @return the count of vertices with outdegree 0
+   */
+  public static int sinkCount(Vertex<Integer> start) {
+    Set<Vertex<Integer>> visited = new HashSet<>();
+    return sinkCount(start, visited); 
+  }
 
-    /**
-     * Returns the count of vertices in a graph that have an outdegree of 0.
-     * 
-     * Returns 0 if the starting vertex is null.
-     * 
-     * @param start the entrypoint to the graph
-     * @return the count of vertices with outdegree 0
-     */
-    public static int sinkCount(Vertex<Integer> start) {
-        return 0;
+  public static int sinkCount(Vertex<Integer> start, Set<Vertex<Integer>> visited) {
+    if(start == null || visited.contains(start)) return 0;
+    visited.add(start);
+    if (start.neighbors == null || start.neighbors.size() == 0) return 1;
+    int count = 0;
+    for (Vertex<Integer> neighbor : start.neighbors) {
+      if (neighbor != null) {
+        int neighborCount = sinkCount(neighbor, visited);
+        count += neighborCount;
+      }
     }
+    return count;
+  }
 }
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