# LeetCode 155. Min Stack

## Description

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.```

## Explanation

We can use two stacks to build a MinStack data structure. One of the stacks is used as a supporting stack to store minimum values in another stack. Whenever we push new element, we push minimum values to the supporting stack.

## Java Solution

``````class MinStack {
Stack<Integer> stack1;
Stack<Integer> stack2;

/** initialize your data structure here. */
public MinStack() {
stack1 = new Stack<>();
stack2 = new Stack<>();
}

public void push(int x) {
stack1.push(x);
if (stack2.isEmpty()) {
stack2.push(x);
} else {
stack2.push(Math.min(stack2.peek(), x));
}
}

public void pop() {
stack1.pop();
stack2.pop();
}

public int top() {
return stack1.peek();
}

public int getMin() {
return stack2.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.getMin();
*/``````

## Python Solution

``````class MinStack:

def __init__(self):
"""
"""
self.stack1 = []
self.stack2 = []

def push(self, x: int) -> None:

self.stack1.append(x)

if not self.stack2:
self.stack2.append(x)
else:
self.stack2.append(min(self.stack2[-1], x))

def pop(self) -> None:
self.stack1.pop()
self.stack2.pop()

def top(self) -> int:
return self.stack1[-1]

def getMin(self) -> int:
return self.stack2[-1]

# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(x)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()``````
• Time Complexity: ~1
• Space Complexity: ~N