LeetCode题解——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 emptyoperations 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:
    // Push element x onto stack.
    void push(int x) {
        if(queue2.empty()) queue1.push(x);
        else queue2.push(x);
    }


    // Removes the element on top of the stack.
    void pop() {
        if(queue2.empty()){
            while(queue1.size()>1){
                queue2.push(queue1.front());
                queue1.pop();
            }
            if(queue1.empty()) throw("stack is empty ");
            else queue1.pop();
        }
        else{
            while(queue2.size()>1){
                queue1.push(queue2.front());
                queue2.pop();
            }
            if(queue2.empty()) throw("stack is empty ");
            else queue2.pop();
        }
    }


    // Get the top element.
    int top() {
        if(queue2.empty()){
            while(queue1.size()>1){
                queue2.push(queue1.front());
                queue1.pop();
            }
            if(queue1.empty()) throw("stack is empty ");
            int top = queue1.front();
            queue2.push(queue1.front());
            queue1.pop();
            return top;
        }
        else{
            while(queue2.size()>1){
                queue1.push(queue2.front());
                queue2.pop();
            }
            if(queue2.empty()) throw("stack is empty ");
            int top = queue2.front();
            queue1.push(queue2.front());
            queue2.pop();
            return top;
        }   
    }


    // Return whether the stack is empty.
    bool empty() {
        if(queue1.empty() && queue2.empty()) return true;
        return false;
    }
private:
    queue<int> queue1;
    queue<int> queue2;
};