232. 用栈实现队列

题目

class MyQueue {
public:
    // 使用两个栈模拟队列
    stack<int> in;
    stack<int> out;
    /** Initialize your data structure here. */
    MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    void push(int x) {
        in.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        // 当out为空时，从in中导入数据并且全部导入
        if (out.empty()) {
            while (!in.empty()) {
                // ！！！使用top返回栈顶的值！！！
                // 栈的 pop 返回 void
                int res = in.top();
                out.push(res);
                in.pop();
            }
        }
        int x = out.top();
        out.pop();
        return x;
    }
    
    /** Get the front element. */
    int peek() {
        int res = this->pop();
        out.push(res);
        return res;
    }
    
    /** Returns whether the queue is empty. */
    bool empty() {
        return in.empty() && out.empty();
    }
};

/**
 * 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();
 */

总结

• 使用top返回栈顶的值
• 栈的 pop 返回 void