chore: implemented functions in Lesson12.java and Stack.java files

Author: mercedes-mathews

lesson_12/structs_java/structs_app/src/main/java/com/codedifferently/lesson12/Lesson12.java

@@ -7,6 +7,25 @@ public class Lesson12 {
    * https://github.com/yang-su2000/Leetcode-algorithm-practice/tree/master/3062-winner-of-the-linked-list-game
    */
   public String gameResult(ListNode head) {
-    return null;
+    int even = 0;
+    int odd = 0;
+    ListNode evenNode = head;
+    ListNode oddNode = (head != null) ? head.next : null;
+    while (evenNode != null) {
+      if (evenNode.val > oddNode.val) {
+        even++;
+      } else {
+        odd++;
+      }
+      evenNode = (evenNode.next != null) ? evenNode.next.next : null;
+      oddNode = (oddNode.next != null) ? oddNode.next.next : null;
+    }
+    if (even > odd) {
+      return "Even";
+    } else if (odd > even) {
+      return "Odd";
+    } else {
+      return "Tie";
+    }
   }
 }

lesson_12/structs_java/structs_app/src/main/java/com/codedifferently/lesson12/Stack.java

@@ -9,18 +9,21 @@ public Stack() {
   }
 
   public void push(int value) {
-    // Your code here
+    ListNode newNode = new ListNode(value, top);
+    top = newNode;
   }
 
   public int pop() {
-    return 0;
+    int value = top.val;
+    top = top.next;
+    return value;
   }
 
   public int peek() {
-    return 0;
+    return top.val;
   }
 
   public boolean isEmpty() {
-    return true;
+    return top == null;
   }
 }