minStack.java

// 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.


//Key insight is to use two stacks

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

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

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

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


//Solution with one stack 

class MinStack {
    int min = Integer.MAX_VALUE;
    Stack<Integer> stack = new Stack<Integer>();
    public void push(int x) {
        // only push the old minimum value when the current 
        // minimum value changes after pushing the new value x
        if(x <= min){          
            stack.push(min);
            min=x;
        }
        stack.push(x);
    }

    public void pop() {
        // if pop operation could result in the changing of the current minimum value, 
        // pop twice and change the current minimum value to the last minimum value.
        if(stack.pop() == min) min=stack.pop();
    }

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

    public int getMin() {
        return min;
    }
}