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.

class MinStack {
    
    Stack<Integer>stack=new Stack<Integer>();
    
    Stack<Integer>min_stack=new Stack<Integer>();
    
    public void push(int x) {
        if(min_stack.isEmpty()||x<=min_stack.peek()){
            min_stack.push(x);
        }
        stack.push(x);
    }

    public void pop() {
        if(stack.peek().equals(min_stack.peek())){
            min_stack.pop();
        }
        stack.pop();
    }

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

    public int getMin() {
        return min_stack.peek();
    }
}

 用一个min_stack来记录当前最小值，以空间换时间的思想。