> For the complete documentation index, see [llms.txt](https://junnie.gitbook.io/nine-chapter/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://junnie.gitbook.io/nine-chapter/8.data-structure/12min-stack.md).

# 155. Min Stack (E)

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

Implement the `MinStack` class:

* `MinStack()` initializes the stack object.
* `void push(int val)` pushes the element `val` onto the stack.
* `void pop()` removes the element on the top of the stack.
* `int top()` gets the top element of the stack.
* `int getMin()` retrieves the minimum element in the stack.

 

**Example 1:**

```
Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2
```

 

**Constraints:**

* `-231 <= val <= 231 - 1`
* Methods `pop`, `top` and `getMin` operations will always be called on **non-empty** stacks.
* At most `3 * 104` calls will be made to `push`, `pop`, `top`, and `getMin`.

### Solution:

使用两个仅支持 `pop` 和 `push` 的栈就可以完成, `stack` 储存压入的数据, `minStack` 储存最小值.

* `push` 直接把元素压入 stack, 对于 minStack, 如果它为空则直接压入, 反之压入当前元素与 minStack 栈顶的最小值
* `pop` 两个栈都弹出一个元素, 返回 stack 弹出的元素
* `min` 返回 minStack 的栈顶

> 还可以令 minStack 为单调栈, 即`push`时只有元素更小的时候才放入这个栈, 而`pop`时只有栈顶与stack栈顶相同时才弹出
>
> 这样可以节约一定的空间, 但是实质上空间复杂度仍然是 O(n), 且多了一些判断, 并不一定更优

```
class MinStack {
    
    private Stack<Integer> stack;
    private Stack<Integer> minStack;

    public MinStack() {
        stack = new Stack<Integer>(); 
        minStack = new Stack<Integer>();
    }
    
    public void push(int val) {
        stack.push(val);
        
        if(minStack.isEmpty())
        {
            minStack.push(val);
        }
        else
        {
            minStack.push(Math.min(minStack.peek(), val));
        }
        
    }
    
    public void pop() {
        stack.pop();
        minStack.pop();
        
    }
    
    public int top() {
        return stack.peek();
        
    }
    
    public int getMin() {
        return minStack.peek();
        
    }
}

```

Version 2: Mono stack

```
public MinStack() {
    stack = new Stack<Integer>();
    minStack = new Stack<Integer>();
}

public void push(int number) {
    stack.push(number);
    if (minStack.empty() == true)
        minStack.push(number);
    else if (minStack.peek() >= number) // 这里考虑的相等的情况也会继续push
        minStack.push(number);
}

public int pop() {
    if (stack.peek().equals(minStack.peek()))
        minStack.pop();
    return stack.pop();
}

public int min() {
    return minStack.peek();
```