Create design-front-middle-back-queue.cpp

Author: kamyu104

C++/design-front-middle-back-queue.cpp

@@ -0,0 +1,75 @@
+// Time:  O(1)
+// Space: O(n)
+
+class FrontMiddleBackQueue {
+public:
+    FrontMiddleBackQueue() {
+        
+    }
+    
+    void pushFront(int val) {
+        left_.emplace_front(val);
+        balance();
+    }
+    
+    void pushMiddle(int val) {
+        if (size(left_) > size(right_)) {
+            right_.emplace_front(left_.back());
+            left_.pop_back();
+        }
+        left_.emplace_back(val);
+    }
+    
+    void pushBack(int val) {
+        right_.emplace_back(val);
+        balance();
+    }
+    
+    int popFront() {
+        int result = -1;
+        if (!empty(left_)) {
+            result = left_.front();
+            left_.pop_front();
+        }
+        balance();
+        return result;
+    }
+    
+    int popMiddle() {
+        int result = -1;
+        if (!empty(left_)) {
+            result = left_.back();
+            left_.pop_back();
+        }
+        balance();
+        return result;
+    }
+    
+    int popBack() {
+        int result = -1;
+        if (!empty(right_)) {
+            result = right_.back();
+            right_.pop_back();
+        } else if (!empty(left_)) {
+            result = left_.back();
+            left_.pop_back();
+        }
+        balance();
+        return result;
+    }
+
+private:
+    void balance() {
+        if (size(left_) > size(right_) + 1) {
+            right_.emplace_front(left_.back());
+            left_.pop_back();
+        } else if (size(left_) < size(right_) ) {
+            left_.emplace_back(right_.front());
+            right_.pop_front();
+        }
+    }
+
+    // using list<int> is also fine but slower
+    deque<int> left_;
+    deque<int> right_;
+};