题目：
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); --> Returns -3.
minStack.pop();
minStack.top(); --> Returns 0.
minStack.getMin(); --> Returns -2.
这道最小栈跟原来的栈相比就是多了一个功能，可以返回该栈的最小值。
class MinStack {
/** initialize your data structure here. */
private Stack<Integer> mStack;
private Stack<Integer> pStack;
public MinStack() {
pStack=new Stack<>();
mStack=new Stack<>();
}
public void push(int x) {
pStack.push(x);
if(mStack.isEmpty()||mStack.peek()>=x){
mStack.push(x);
}
}
public void pop() {
int x=pStack.pop();
if(x==mStack.peek())
mStack.pop();
}
public int top() {
return pStack.peek();
}
public int getMin() {
return mStack.peek();
}
}
