Create 1670.Design-Front-Middle-Back-Queue_v2.cpp

Linked_List/1670.Design-Front-Middle-Back-Queue/1670.Design-Front-Middle-Back-Queue_v2.cpp

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+class FrontMiddleBackQueue {
+    list<int> List;
+    list<int>::iterator mid;
+    int n = 0;
+public:
+    FrontMiddleBackQueue() {        
+    }
+
+    void pushFront(int val) 
+    {                
+        List.push_front(val);
+        if (n==0)
+            mid = List.begin();
+        else if (n%2==1)
+            mid = prev(mid);
+        n++;
+    }    
+
+    void pushMiddle(int val) 
+    {                
+        if (n==0)
+        {
+            List.push_back(val);
+            mid = List.begin();
+        }            
+        else if (n%2==0)
+        {
+            List.insert(next(mid), val);
+            mid = next(mid);
+        }
+        else
+        {
+            List.insert(mid, val);
+            mid = prev(mid);
+        }
+        n++;            
+    }
+
+    void pushBack(int val) 
+    {
+        List.push_back(val);        
+        if (n==0)
+            mid = List.begin();
+        else if (n%2==0)
+            mid = next(mid);
+        n++;
+    }
+
+    int popFront() 
+    {
+        if (n==0) return -1;
+        int ret = List.front();
+        if (n%2==0)
+            mid = next(mid);        
+        List.pop_front();
+        n--;            
+        return ret;
+    }
+
+    int popMiddle() 
+    {
+        if (n==0) return -1;
+        int ret = *mid;                     
+        list<int>::iterator mid_new;
+        if (n%2==1)        
+            mid_new = prev(mid);                    
+        else
+            mid_new = next(mid);
+        List.erase(mid);
+        n--;   
+        mid = mid_new;
+        return ret;
+    }
+
+    int popBack() 
+    {
+        if (n==0) return -1;
+        int ret = List.back();        
+        if (n%2==1)
+            mid = prev(mid);
+        List.pop_back();
+        n--;
+        return ret;
+    }
+};
+
+/**
+ * Your FrontMiddleBackQueue object will be instantiated and called as such:
+ * FrontMiddleBackQueue* obj = new FrontMiddleBackQueue();
+ * obj->pushFront(val);
+ * obj->pushMiddle(val);
+ * obj->pushBack(val);
+ * int param_4 = obj->popFront();
+ * int param_5 = obj->popMiddle();
+ * int param_6 = obj->popBack();
+ */