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).

用栈实现一个队列

思路

设置两个栈，插入时，将栈1元素转存到栈2，然后新元素入栈1，再将栈2元素添加到栈1中，栈2清空

代码

class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}

/** Push element x to the back of queue. */
void push(int x) {
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
s1.push(x);
while(!s2.empty()){
s1.push(s2.top());
s2.pop();
}
}

/** Removes the element from in front of queue and returns that element. */
int pop() {
int result = s1.top();
s1.pop();
return result;
}

/** Get the front element. */
int peek() {
return s1.top();
}

/** Returns whether the queue is empty. */
bool empty() {
return s1.empty();
}
private:
stack<int> s1, s2;
};