LeetCode（232） Implement Queue using Stacks

题目

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).

分析

用栈实现队列；

只需要两个辅助栈，一个保存压入元素，一个保存弹出元素；当弹出元素栈空时，把所有入栈元素压入出栈；

AC代码

class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
inStack.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if (outStack.empty())
{
while (!inStack.empty())
{
outStack.push(inStack.top());
inStack.pop();
}
}
outStack.pop();
}

// Get the front element.
int peek(void) {
if (outStack.empty())
{
while (!inStack.empty())
{
outStack.push(inStack.top());
inStack.pop();
}
}
return outStack.top();
}

// Return whether the queue is empty.
bool empty(void) {
if (outStack.empty() && inStack.empty())
return true;
return false;
}

private:
stack<int> inStack;
stack<int> outStack;
};

GitHub测试程序源码