【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).

两个栈实现堆是经典算法。

进队时，压入stk1。

出队时，stk2弹出。

stk2为空时，stk1倒入stk2。两次逆序恢复了原序。

class Queue {
public:
stack<int> stk1;    // push
stack<int> stk2;    // pop

// Push element x to the back of queue.
void push(int x) {
stk1.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(stk2.empty())
{
while(!stk1.empty())
{
int top = stk1.top();
stk1.pop();
stk2.push(top);
}
}
stk2.pop();
}

// Get the front element.
int peek(void) {
if(stk2.empty())
{
while(!stk1.empty())
{
int top = stk1.top();
stk1.pop();
stk2.push(top);
}
}
return stk2.top();
}

// Return whether the queue is empty.
bool empty(void) {
return stk1.empty()&&stk2.empty();
}
};