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.

Solution: one stack

class MinStack {
    int min;
    Stack stack;
    
    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() {
        if(stack.isEmpty())
            return;
        int temp = stack.pop();
        if(temp == min && !stack.isEmpty()) {
            min = stack.pop();
        }
    }

    public int top() {
        if(stack.isEmpty()) {
            return -1;
        }
        return stack.peek();
    }

    public int getMin() {
        return min;
    }
}