Leetcode-Implement Queue using Stacks（C++）

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).

题意：利用stack来实现queue操作。

解题思路：关键在于将新插入的数值永远排在queue的前面。

利用两个stack，一个空stack0，将另外一个stack里面的数值依次pop然后存入stack0中，然后再将要插入的数值放入stack0中。再依次将stack0中的数值pop出重新放入原来的stack中。

class Queue {
public:
stack<int> s[2];
int t=1;
// Push element x to the back of queue.
void push(int x) {
if(s[1].empty())
{
s[1].push(x);
}
else
{
while(!s[1].empty())
{
s[0].push(s[1].top());
s[1].pop();
}
s[0].push(x);
while(!s[0].empty())
{
s[1].push(s[0].top());
s[0].pop();
}

}

}

// Removes the element from in front of queue.
void pop(void) {
if(!s[1].empty())
{
s[1].pop();
}

}

// Get the front element.
int peek(void) {
if(!s[1].empty())
{
return s[1].top();
}

}

// Return whether the queue is empty.
bool empty(void) {
return s[1].empty();

}
};