使用栈实现队列：
将栈中的元素取出再存入一次就是队列：
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
s.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
stack<int> s1;
while(!s.empty())
{
int t=s.top();
s.pop();
s1.push(t);
}
int t=s1.top();
s1.pop();
while(!s1.empty())
{
int t=s1.top();
s.push(t);
s1.pop();
}
return t;
}
/** Get the front element. */
int peek() {
stack<int> s_tem=s;
while(s_tem.size()>1)
{
s_tem.pop();
}
return s_tem.top();
}
/** Returns whether the queue is empty. */
bool empty() {
return s.empty();
}
private:
stack<int> s;
};
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* bool param_4 = obj.empty();
*/
更简单一点实现，pop和top的实现互相借助时：
class MyQueue {
public:
stack<int> S1, S2;
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
S1.push(x);
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
int v = -1;
if (!S2.empty()) { v = S2.top(); S2.pop();}
else {
while(!S1.empty()) {
S2.push(S1.top());
S1.pop();
}
v = S2.top();
S2.pop();
}
return v;
}
/** Get the front element. */
int peek() {
if (!S2.empty()) return S2.top();
else {
while(!S1.empty()) {
S2.push(S1.top());
S1.pop();
}
return S2.top();
}
}
/** Returns whether the queue is empty. */
bool empty() {
return (S1.empty()) && (S2.empty());
}
};
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* bool param_4 = obj.empty();
*/