adding the queue stack problems

lectures/21_hash_tables_II/README.md

@@ -164,3 +164,5 @@ but requires more work to get right.
 
 - [Leetcode problem 1451: Rearrange Words in a Sentence](https://leetcode.com/problems/rearrange-words-in-a-sentence/). Solution: [p1451_rearrange_words_in_a_sentence.cpp](../../leetcode/p1451_rearrange_words_in_a_sentence.cpp).
 - [Leetcode problem 508: Most Frequent Subtree Sum](https://leetcode.com/problems/most-frequent-subtree-sum/). Solution: [p508_most_frequent_subtree_sum.cpp](../../leetcode/p508_most_frequent_subtree_sum.cpp).
+- [Leetcode problem 225: Implement Stack using Queues](https://leetcode.com/problems/implement-stack-using-queues/). Solution: [p225_stack_using_queues.cpp](../../leetcode/p225_stack_using_queues.cpp).
+- [Leetcode problem 232: Implement Queue using Stacks](https://leetcode.com/problems/implement-queue-using-stacks/). Solution: [p232_queue_using_stacks.cpp](../../leetcode/p232_queue_using_stacks.cpp).

leetcode/p225_stack_using_queues.cpp

@@ -0,0 +1,40 @@
+class MyStack {
+public:
+    MyStack() {
+        
+    }
+    
+    void push(int x) {
+        que.push(x);
+        int size = que.size();
+        for(int i=0;i<size-1;i++){
+            que.push(que.front());
+            que.pop();
+        }
+    }
+    
+    int pop() {
+        int x = que.front();
+        que.pop();
+        return x;
+    }
+    
+    int top() {
+        return que.front();
+    }
+    
+    bool empty() {
+        return que.empty();
+    }
+private:
+    std::queue<int> que;
+};
+
+/**
+ * Your MyStack object will be instantiated and called as such:
+ * MyStack* obj = new MyStack();
+ * obj->push(x);
+ * int param_2 = obj->pop();
+ * int param_3 = obj->top();
+ * bool param_4 = obj->empty();
+ */

leetcode/p232_queue_using_stacks.cpp

@@ -0,0 +1,52 @@
+// leetcode 232: implementing queues using stacks
+
+class MyQueue {
+public:
+    MyQueue() {
+        
+    }
+    
+    // push 1
+    // push 2
+    // peek
+    void push(int x) {
+        int size = stack1.size();
+        // move every element in stack1 into stack2
+        for(int i=0;i<size;i++){
+            stack2.push(stack1.top());
+            stack1.pop();
+        }
+        stack1.push(x);
+        // move every element in stack2 into stack1
+        for(int i=0;i<size;i++){
+            stack1.push(stack2.top());
+            stack2.pop();
+        }
+    }
+    
+    int pop() {
+        int x = stack1.top();
+        stack1.pop();
+        return x;
+    }
+    
+    int peek() {
+        return stack1.top();
+    }
+    
+    bool empty() {
+        return stack1.empty();
+    }
+private:
+    std::stack<int> stack1;
+    std::stack<int> stack2;
+};
+
+/**
+ * Your MyQueue object will be instantiated and called as such:
+ * MyQueue* obj = new MyQueue();
+ * obj->push(x);
+ * int param_2 = obj->pop();
+ * int param_3 = obj->peek();
+ * bool param_4 = obj->empty();
+ */