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.

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();
}
};