# 0155. Min Stack

<https://leetcode.com/problems/min-stack>

## Description

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`.

## ac1: two stack

```java
class MinStack {
    private Stack<Integer> stack;
    private Stack<Integer> stack2;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        stack2 = new Stack<>();
    }

    public void push(int x) {
        stack.push(x);
        if (stack2.isEmpty() || x <= stack2.peek()) {
            stack2.push(x);
        }
    }

    public void pop() {
        int tmp = stack.pop();
        if (tmp == stack2.peek()) {
            stack2.pop(); 
        }
    }

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

    public int getMin() {
        return stack2.peek();
    }
}
```

## ac2: one stack

Key: in the stack, add some roadmark to indicate min from now on.ß

```java
class MinStack {
    private Stack<Integer> stack;
    private int min;

    /** initialize your data structure here. */
    public MinStack() {
        stack = new Stack<>();
        min = Integer.MAX_VALUE;
    }

    public void push(int x) {
        if (x <= min) {
            stack.push(min);
            min = x;
        }
        stack.push(x);
    }

    public void pop() {
        int tmp = stack.pop();
        if (tmp == min) {
            min = stack.pop();
        }
    }

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

    public int getMin() {
        return min;
    }
}
```