fix: handle single-node pops in linked-list deques (#1824)

codes/csharp/chapter_stack_and_queue/linkedlist_deque.cs

@@ -74,26 +74,29 @@ public class LinkedListDeque {
         if (IsEmpty())
             throw new Exception();
         int? val;
+        // 队列长度为 1 时, 头尾指向同一节点
+        if (queSize == 1)
+        {
+            val = front!.val;
+            front = null;
+            rear = null;
+        }
         // 队首出队操作
-        if (isFront) {
-            val = front?.val; // 暂存头节点值
+        else if (isFront) {
+            val = front!.val; // 暂存头节点值
             // 删除头节点
             ListNode? fNext = front?.next;
-            if (fNext != null) {
-                fNext.prev = null;
-                front!.next = null;
-            }
+            fNext!.prev = null;
+            front.next = null;
             front = fNext;   // 更新头节点
         }
         // 队尾出队操作
         else {
-            val = rear?.val;  // 暂存尾节点值
+            val = rear!.val;  // 暂存尾节点值
             // 删除尾节点
             ListNode? rPrev = rear?.prev;
-            if (rPrev != null) {
-                rPrev.next = null;
-                rear!.prev = null;
-            }
+            rPrev!.next = null;
+            rear.prev = null;
             rear = rPrev;    // 更新尾节点
         }