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：

用两个栈来维护即可。

代码：

class Queue {
stack<int> stkfront;
stack<int> stkback;
public:
// Push element x to the back of queue.
void push(int x) {
stkfront.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(!stkfront.empty()){
while(!stkfront.empty()){
stkback.push(stkfront.top());
stkfront.pop();
}
stkback.pop();
while(!stkback.empty()){
stkfront.push(stkback.top());
stkback.pop();
}
}
else exit(1);
}

// Get the front element.
int peek(void) {
if(!stkfront.empty()){
while(!stkfront.empty()){
stkback.push(stkfront.top());
stkfront.pop();
}
int res=stkback.top();
while(!stkback.empty()){
stkfront.push(stkback.top());
stkback.pop();
}
return res;
}
else return NULL;
}

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