# 155. Min Stack

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

• push(x) — Push element x onto stack.
• pop() — Removes the element on top of the stack.
• top() — Get the top element.
• getMin() — Retrieve the minimum element in the stack.

Example:

```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.
```
这道题看上去简单，但是注意在出栈的时候，如果min的出去了，怎么破？

```class MinStack {
Stack<Integer> s = new Stack<Integer>();
Stack<Integer> ms = new Stack<Integer>();
int min = Integer.MAX_VALUE;
/** initialize your data structure here. */
public MinStack() {

}

public void push(int x) {
s.push(x);
if(min>x){
min = x;
}
ms.push(min);
}

public void pop() {
s.pop();
ms.pop();
if(s.isEmpty()){
min = Integer.MAX_VALUE;
}
else{
min = ms.peek();
}

}

public int top() {
return s.peek();
}

public int getMin() {
return min;
}
}

/**
* 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();
*/```

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