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Java-based LeetCode algorithm problem solutions, regularly updated

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package g0101_0200.s0155_min_stack;

// #Easy #Top_100_Liked_Questions #Top_Interview_Questions #Stack #Design
// #Data_Structure_II_Day_14_Stack_Queue #Programming_Skills_II_Day_18 #Level_2_Day_16_Design
// #Udemy_Design #Big_O_Time_O(1)_Space_O(N) #2022_06_25_Time_3_ms_(100.00%)_Space_44.3_MB_(85.39%)

/**
 * 155 - Min Stack\.
 *
 * Easy
 *
 * Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.
 *
 * Implement the `MinStack` class:
 *
 * *   `MinStack()` initializes the stack object.
 * *   `void push(int val)` pushes the element `val` onto the stack.
 * *   `void pop()` removes the element on the top of the stack.
 * *   `int top()` gets the top element of the stack.
 * *   `int getMin()` retrieves the minimum element in the stack.
 *
 * **Example 1:**
 *
 * **Input**
 *
 *     ["MinStack","push","push","push","getMin","pop","top","getMin"]
 *     [ [],[-2],[0],[-3],[],[],[],[]]
 *
 * **Output:** [null,null,null,null,-3,null,0,-2]
 *
 * **Explanation:**
 *
 *     MinStack minStack = new MinStack();
 *     minStack.push(-2);
 *     minStack.push(0);
 *     minStack.push(-3);
 *     minStack.getMin(); // return -3
 *     minStack.pop();
 *     minStack.top(); // return 0
 *     minStack.getMin(); // return -2 
 *
 * **Constraints:**
 *
 * *   -2³¹ <= val <= 2³¹ - 1
 * *   Methods `pop`, `top` and `getMin` operations will always be called on **non-empty** stacks.
 * *   At most 3 * 10⁴ calls will be made to `push`, `pop`, `top`, and `getMin`.
**/
public class MinStack {
    private static class Node {
        int min;
        int data;
        Node nextNode;
        Node previousNode;

        public Node(int min, int data, Node previousNode, Node nextNode) {
            this.min = min;
            this.data = data;
            this.previousNode = previousNode;
            this.nextNode = nextNode;
        }
    }

    private Node currentNode;

    // initialize your data structure here.
    public MinStack() {
        // no initialization needed.
    }

    public void push(int val) {
        if (currentNode == null) {
            currentNode = new Node(val, val, null, null);
        } else {
            currentNode.nextNode = new Node(Math.min(currentNode.min, val), val, currentNode, null);
            currentNode = currentNode.nextNode;
        }
    }

    public void pop() {
        currentNode = currentNode.previousNode;
    }

    public int top() {
        return currentNode.data;
    }

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

/*
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(val);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */