Completed Design-2 [2024] Problem4

2024_Problem4_Impl_Queue_using_Stack.java

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+//232. Implement Queue using Stacks - https://leetcode.com/problems/implement-queue-using-stacks/description/
+//Time Complexity: All operation takes constant time O(1)
+//Space Complexity: O(n)
+
+class MyQueue {
+
+    Stack<Integer> stack;
+    Stack<Integer> reverseStack;
+
+    public MyQueue() {
+        this.stack = new Stack<>();
+        this.reverseStack = new Stack<>();
+    }
+
+    public void push(int x) {
+        stack.push(x);                      // Push element x to the end
+    }
+
+    private void reverseStack(){
+        while(!stack.empty()){
+            reverseStack.push(stack.pop()); //pop elements from main stack and push in reverseStack in reverse order
+        }
+    }
+
+    /** Removes the element from in front of queue and returns that element. */
+    public int pop() {
+        int popElement = -1;                //default value of stack if empty
+
+        if(reverseStack.empty())
+            reverseStack();                 //populate the reverse stack
+
+        popElement = reverseStack.pop();    //poping the first element in queue
+        return popElement;
+    }
+
+    /** Get the front element. */
+    public int peek() {
+        int peekElement = -1;               //default value if stack is empty
+
+        if(reverseStack.empty())
+            reverseStack();                 //populate the reverse stack
+
+        peekElement = reverseStack.peek();  //peeking the first element in queue w/o poping
+        return peekElement;
+    }
+
+    /** Returns whether the queue is empty. */
+    public boolean empty() {
+        return (stack.empty() && reverseStack.empty());
+    }
+}