feat: add lesson_12 extra credit

Author: txtran224

lesson_12/structs_ts/src/lesson12.ts

@@ -6,6 +6,22 @@ export class Lesson12 {
    * https://github.com/yang-su2000/Leetcode-algorithm-practice/tree/master/3062-winner-of-the-linked-list-game
    */
   public gameResult(head: ListNode | null): string {
-    return '';
+    let even = 0;
+    let odd = 0;
+    while (head != null && head.next != null) {
+      if (head.val > head.next.val) {
+        even++;
+      } else {
+        odd++;
+      }
+      head = head.next.next !== undefined ? head.next.next : null;
+    }
+    if (even === odd) {
+      return 'Tie';
+    } else if (even > odd) {
+      return 'Even';
+    } else {
+      return 'Odd';
+    }
   }
 }

lesson_12/structs_ts/src/stack.ts

@@ -8,18 +8,25 @@ export class Stack {
   }
 
   push(value: number): void {
-    throw new Error('Not implemented');
+    const newnode = new ListNode(value);
+    newnode.next = this.top;
+    this.top = newnode;
   }
 
   pop(): number | undefined {
-    throw new Error('Not implemented');
+    const nodevalue = this.top?.val;
+    this.top = this.top?.next;
+    return nodevalue;
   }
 
   peek(): number | null {
-    throw new Error('Not implemented');
+    if (this.top !== null && this.top !== undefined) {
+      return this.top.val;
+    }
+    return null;
   }
 
   isEmpty(): boolean {
-    throw new Error('Not implemented');
+    return this.top == null;
   }
 }