leetcode-232-Implement Queue using Stacks

                              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.

• 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).

用栈实现队列。

两个栈即可实现队列。

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) {
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        s2.pop();
        while (!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }
    }

    // Get the front element.
    int peek(void) {
        while (!s1.empty()) {
            s2.push(s1.top());
            s1.pop();
        }
        int x = s2.top();//取栈顶元素
        while (!s2.empty()) {
            s1.push(s2.top());
            s2.pop();
        }        
        return x;
    }

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

原文：http://blog.csdn.net/u014705854/article/details/47259359