feat: implement game result logic and enhance stack operations

Author: bryanasb251

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

@@ -7,6 +7,19 @@ 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 odd = 0, even = 0;
+    for (; head != null; head = head.next.next) {
+      int a = head.val;
+      int b = head.next.val;
+      odd += a < b ? 1 : 0;
+      even += a > b ? 1 : 0;
+    }
+    if (odd > even) {
+      return "Odd";
+    }
+    if (odd < even) {
+      return "Even";
+    }
+    return "Tie";
   }
 }

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

@@ -10,17 +10,40 @@ public Stack() {
 
   public void push(int value) {
     // Your code here
+    ListNode newNode = new ListNode(value);
+    if (top == null) {
+      top = newNode;
+    } else {
+      ListNode temp = top;
+      top = newNode;
+      top.next = temp;
+    }
   }
 
   public int pop() {
-    return 0;
+    if (top != null) {
+      int value = top.val;
+      top = top.next;
+      return value;
+    } else {
+      return -1;
+    }
   }
 
   public int peek() {
-    return 0;
+    if (top != null) {
+      return top.val;
+    } else {
+      throw new RuntimeException("Stack is empty");
+        
+    }
   }
 
   public boolean isEmpty() {
-    return true;
+    if (top == null) {
+      return true;
+    } else {
+      return false;
+    }
   }
 }