easy的棧，155取最小的元素，用兩個棧，第二個棧存當前的棧的最小元素，然後彈出的時候根據情況彈出元素：
class MinStack {
public:
/** initialize your data structure here. */
MinStack() {
}
void push(int x) {
s1.push_back(x);
if(s2.empty()||x<=getMin())
{
s2.push_back(x);
}
}
void pop() {
if(s1.back()==getMin())
{
s2.pop_back();
}
s1.pop_back();
}
int top() {
return s1.back();
}
int getMin() {
return s2.back();
}
vector<int> s1;
vector<int> s2;
};
/**
* Your MinStack object will be instantiated and called as such:
* MinStack obj = new MinStack();
* obj.push(x);
* obj.pop();
* int param_3 = obj.top();
* int param_4 = obj.getMin();
*/255用deque，雙向隊列：
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
s1.push_back(x);
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int temp=top();
s1.pop_back();
return temp;
}
/** Get the top element. */
int top() {
return s1.back();
}
/** Returns whether the stack is empty. */
bool empty() {
if(s1.empty())
{
return true;
}
return false;
}
deque<int> s1;
};
/**
* Your MyStack object will be instantiated and called as such:
* MyStack obj = new MyStack();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.top();
* bool param_4 = obj.empty();
*/232，用兩個棧實現一個隊列，容易想的就是s2緩存，s1是主要的，push直接在s1後面push就好，但是pop要先倒到s2，再倒回s1，這裏倒的時候可以少倒一個，直接pop（這個代碼沒有體現）：
class MyQueue {
public:
/** 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() {
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
int temp=s2.top();
s2.pop();
while(!s2.empty())
{
s1.push(s2.top());
s2.pop();
}
return temp;
}
/** Get the front element. */
int peek() {
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
int temp=s2.top();
while(!s2.empty())
{
s1.push(s2.top());
s2.pop();
}
return temp;
}
/** Returns whether the queue is empty. */
bool empty() {
if(s1.empty())
{
return true;
}
return false;
}
stack<int> s1;
stack<int> s2;
};
/**
* 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();
*/另一種變種，s2負責彈出頭的，s1負責push尾的，一次倒一個，不用倒過去再倒回來，如果push，發現元素都在s2裏，就倒回來，否則不用；如果pop，元素都在s1裏就倒到s2，否則不用：
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
if(s2.empty())
s1.push(x);
else
{
while(!s2.empty())
{
s1.push(s2.top());
s2.pop();
}
s1.push(x);
}
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
int temp;
if(s1.empty())
{
temp=s2.top();
s2.pop();
}
else
{
int n=s1.size();
for(int i=0;i<n-1;i++)
{
s2.push(s1.top());
s1.pop();
}
temp=s1.top();
s1.pop();
}
return temp;
}
/** Get the front element. */
int peek() {
int temp;
if(s1.empty())
{
temp=s2.top();
}
else
{
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
temp=s2.top();
}
return temp;
}
/** Returns whether the queue is empty. */
bool empty() {
if(s1.empty()&&s2.empty())
{
return true;
}
return false;
}
stack<int> s1;
stack<int> s2;
};
/**
* 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();
*/最簡單的變種，只要push就到s1裏，pop的時候，s2有元素就從s2裏面pop，如果s2中沒有元素了，再把s1中的元素都倒到s2裏，然後pop，這種倒的次數最少。
class MyQueue {
public:
/** 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 temp;
if(!s2.empty())
{
temp=s2.top();
s2.pop();
}
else
{
int n=s1.size();
for(int i=0;i<n-1;i++)
{
s2.push(s1.top());
s1.pop();
}
temp=s1.top();
s1.pop();
}
return temp;
}
/** Get the front element. */
int peek() {
int temp;
if(!s2.empty())
{
temp=s2.top();
}
else
{
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
temp=s2.top();
}
return temp;
}
/** Returns whether the queue is empty. */
bool empty() {
if(s1.empty()&&s2.empty())
{
return true;
}
return false;
}
stack<int> s1;
stack<int> s2;
};
/**
* 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();
*/可參考：http://www.cnblogs.com/wanghui9072229/archive/2011/11/22/2259391.html
