LeetCode题解——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 emptyoperations 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).

• class Queue {public:    // Push element x to the back of queue.    void push(int x) {        stack1.push(x);    }    // Removes the element from in front of queue.    void pop(void) {        if(!stack2.empty()) stack2.pop();        else{            while(!stack1.empty()){                stack2.push(stack1.top());                stack1.pop();            }            if(stack2.empty()) throw(" Queue is empty! ");            stack2.pop();        }    }    // Get the front element.    int peek(void) {        if(!stack2.empty()) return stack2.top();        else{             while(!stack1.empty()){                stack2.push(stack1.top());                stack1.pop();            }            if(stack2.empty()) throw(" Queue is empty! ");            return stack2.top();        }    }    // Return whether the queue is empty.    bool empty(void) {        if(stack1.empty() && stack2.empty()) return true;        return false;    }private:    stack<int> stack1;    stack<int> stack2;};