用栈实现队列-用队列实现栈

用栈实现队列

leetcode ：

Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push
to top, peek/pop from top, size,
  and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

思路：用两个栈，s1,s2，s2只是为了移动数据用的。

class Queue {
public:
    stack<int> s1,s2;
    // Push element x to the back of queue.
    void push(int x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.
    void pop(void) {
        //将s1内元素除了最后一个全部移动到s2
        while(s1.size()>1)
        {
            s2.push(s1.top());
            s1.pop();
        }

        //最后一个元素删除
        s1.pop();

        //将s2内元素全部移动到s1
        while(s2.size()>0)
        {
            s1.push(s2.top());
            s2.pop();
        }
    }

    // Get the front element.
    int peek(void) {
          //将s1内元素除了最后一个全部移动到s2
        while(s1.size()>1)
        {
            s2.push(s1.top());
            s1.pop();
        }

        //保存最后一个元素，然后再移动到s2
        int res=s1.top();
        s2.push(res);
        s1.pop();

        //将s2内元素全部移动到s1
        while(s2.size()>0)
        {
            s1.push(s2.top());
            s2.pop();
        }
        return res;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return s1.empty();
    }
};

leetcode:

Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push
to back, peek/pop from front, size,
  and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

class Stack {
public:
    deque<int> q1,q2;
    // Push element x onto stack.
    void push(int x) {
        q1.push_back(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        while(q1.size()>1)//将q1数据除了最后一个外，全部移动到q2
        {
            q2.push_back(q1.front());
            q1.pop_front();
        }

        q1.pop_front();//删除栈顶元素

        while(q2.size()>0)//将q2元素全部移动回到q1
        {
            q1.push_back(q2.front());
            q2.pop_front();
        }
    }

    // Get the top element.
    int top() {
        while(q1.size()>1)//将q1数据除了最后一个外，全部移动到q2
        {
            q2.push_back(q1.front());
            q1.pop_front();
        }

        //获取栈顶元素，将其移动到q2
        int res=q1.front();
        q2.push_back(q1.front());
        q1.pop_front();

        while(q2.size()>0)//将q2元素全部移动回到q1
        {
            q1.push_back(q2.front());
            q2.pop_front();
        }

        return res;
    }

    // Return whether the stack is empty.
    bool empty() {
        return q1.empty();
    }
};

版权声明：本文为博主原创文章，未经博主允许不得转载。