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.

• Time: O(n) Space: O(1)

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