From beebebcebb5de92dfdb724046c8561bdc6cbf7dc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=94=90=E6=A0=91=E6=A3=AE?= <14021051@buaa.edu.cn>
Date: Tue, 16 Oct 2018 22:17:21 +0800
Subject: [PATCH] Create 225. Implement Stack using Queues.md

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+# [225. Implement Stack using Queues](https://leetcode.com/problems/implement-stack-using-queues/description/)
+# 思路
+用队列实现栈。   
+栈是先进后出，队列是后进先出，所以要想用队列实现栈时为了返回栈顶元素就得完整pop一遍队列里的元素，或者在进队的时候就将栈顶元素移到队头，下面的代码采用后者思路。
+因为pop和top都要求找到栈顶元素，采用前者思路的话会产生混乱。    
+注意学习stl中queue的一些操作：push、pop、front
+# C++
+```
+class MyStack {
+private:
+    queue<int>q;
+public:
+    /** Initialize your data structure here. */
+    MyStack() {
+    }
+    
+    /** Push element x onto stack. */
+    void push(int x) {  // 先将x进队，然后再将x前面的元素依次pop出来并入队尾，这样x就位于队头了。
+        q.push(x);
+        int tmp;
+        for(int i = 0; i < q.size() - 1; i++){
+            tmp = q.front();
+            q.pop();
+            q.push(tmp);
+        }
+    }
+    
+    /** Removes the element on top of the stack and returns that element. */
+    int pop() {
+        int res = q.front();
+        q.pop();
+        return res;
+    }
+    
+    /** Get the top element. */
+    int top() {
+        return q.front();    
+    }
+    
+    /** Returns whether the stack is empty. */
+    bool empty() {
+        return q.empty();
+    }
+};
+```