題目來源
使用隊列來實現(xiàn)棧，我一開始先入為主的想法就是用兩個隊列來實現(xiàn)，push進一個，就把這個元素push進一個空隊列，然后把另一個隊列的元素一次出對再入剛push進一個元素的隊列。還得判斷一下當前哪個隊列為空，冗長又復雜。
代碼如下：

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        cur = 1;
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        if (cur == 1) {
            q1.push(x);
            while (!q2.empty()) {
                int tmp = q2.front();
                q2.pop();
                q1.push(tmp);
            }
            cur = 2;
        }
        else {
            q2.push(x);
            while (!q1.empty()) {
                int tmp = q1.front();
                q1.pop();
                q2.push(tmp);
            }
            cur = 1;
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        if (cur == 1 && !q2.empty()) {
            int tmp = q2.front();
            q2.pop();
            return tmp;
        }
        if (cur == 2 && !q1.empty()) {
            int tmp = q1.front();
            q1.pop();
            return tmp;
        }
        return 0;
    }
    
    /** Get the top element. */
    int top() {
        if (cur == 1 && !q2.empty()) {
            return q2.front();
        }
        if (cur == 2 && !q1.empty()) {
            return q1.front();
        }
        return 0;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        if (q1.empty() && q2.empty())
            return true;
        return false;
    }
private:
    int cur;
    queue<int> q1;
    queue<int> q2;
};

/**
 * 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();
 */

然后實際上一個隊列就可以了，每次push的時候先push，然后把之前的出隊列再入隊列。代碼如下：

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        q.push(x);
        for (int i=0; i<q.size()-1; i++) {
            int tmp = q.front();
            q.pop();
            q.push(tmp);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int tmp = q.front();
        q.pop();
        return tmp;
    }
    
    /** Get the top element. */
    int top() {
        return q.front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q.empty();
    }
private:
    int cur;
    queue<int> q;
};

/**
 * 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();
 */