leetcode代码记录 232 Implement Queue using Stacks

题目如下：

232. 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).

解题思想，使用两个栈代替队列，一个进，一个出。代码如下：

class Queue {
public:

stack<int> stkin;
stack<int> stkout;
// Push element x to the back of queue.
void push(int x) {
stkin.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
if(stkout.empty()) {
if(stkin.empty()) {
return ;
}
int size = stkin.size();
for(int i = 0; i < size; i++) {
stkout.push(stkin.top());
stkin.pop();
}
}
stkout.pop();
}

// Get the front element.
int peek(void) {
if(stkout.empty()) {
if(stkin.empty()) {
return 0;
}
int size = stkin.size();
for(int i = 0; i < size; i++) {
stkout.push(stkin.top());
stkin.pop();
}
}
return (stkout.top());
}

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