Create 232. Implement Queue using Stacks.md

232. Implement Queue using Stacks.md

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+# [232. Implement Queue using Stacks](https://leetcode.com/problems/implement-queue-using-stacks/description/)
+# 思路
+用栈实现队列。   
+设置两个栈stk1和stk2，stk1中的元素是按照正常入栈顺序排的，stk2则是逆序，且同一时刻stk1和stk2至少一个为空。     
+若stk1不空，对队列入队的话直接对stk1入栈即可，否则要将stk2中所有元素pop到stk1中后再对stk1入栈。   
+若stk2不空，对队列出队的话直接对stk2出栈即可，否则要将stk1中所有元素pop到stk2中后再对stk2出栈。
+# C++
+```
+class MyQueue {
+private:
+    stack<int>stk1;
+    stack<int>stk2;
+public:
+    /** Initialize your data structure here. */
+    MyQueue() {
+        
+    }
+    /** Push element x to the back of queue. */
+    void push(int x) {
+        if(stk1.empty()){
+            int tmp;
+            while(!stk2.empty()){
+                tmp = stk2.top();
+                stk2.pop();
+                stk1.push(tmp);
+            }
+        }
+        stk1.push(x);
+    }
+    /** Removes the element from in front of queue and returns that element. */
+    int pop() {
+        int tmp;
+        if(stk2.empty()){
+            while(!stk1.empty()){
+                tmp = stk1.top();
+                stk1.pop();
+                stk2.push(tmp);
+            }
+        }
+        tmp = stk2.top();
+        stk2.pop();
+        return tmp;
+    }
+    
+    /** Get the front element. */
+    int peek() {
+        if(stk2.empty()){
+            int tmp;
+            while(!stk1.empty()){
+                tmp = stk1.top();
+                stk1.pop();
+                stk2.push(tmp);
+            }
+        }
+        return stk2.top();
+    }
+    /** Returns whether the queue is empty. */
+    bool empty() {
+        return stk1.empty() && stk2.empty();
+    }
+};
+```