定义栈的数据结构，请在该类型中实现一个能够得到栈的最小元素的 min 函数在该栈中，调用 min、push 及 pop 的时间复杂度都是 O(1)。
示例:
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.min(); --> 返回 -3.
minStack.pop();
minStack.top(); --> 返回 0.
minStack.min(); --> 返回 -2.
提示：
各函数的调用总次数不超过 20000 次
思路：辅助栈
代码：
class MinStack {
Stack<Integer> stack;
Stack<Integer> minstack;
/** initialize your data structure here. */
public MinStack() {
stack = new Stack();
minstack = new Stack();
}
public void push(int x) {
stack.push(x);
if(minstack.isEmpty()||x<=minstack.peek()){
minstack.push(x);
}else{
minstack.push(minstack.peek());
}
}
public void pop() {
stack.pop();
minstack.pop();
}
public int top() {
return stack.peek();
}
public int min() {
return minstack.peek();
}
}
/**
* 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.min();
*/
