https://leetcode.com/problems/min-stack/description/

minStackTwoStacks.java

class MinStackNode {
    public int val;
    public int curMin;

    public MinStackNode(int val, int curMin) {
        this.val = val;
        this.curMin = curMin;
    }
}

/** Below solution is simply define a node keeps track of current minimum, saves space! **/
private Stack<MinStackNode> minStackWithNode = new Stack<>();

public void push(int x) {
    if (minStackWithNode.isEmpty()) {
        minStackWithNode.push(new MinStackNode(x, x));
        return;
    }

    if (x <= minStackWithNode.peek().curMin) {
        minStackWithNode.push(new MinStackNode(x, x));
    } else {
        minStackWithNode.push(new MinStackNode(x, minStackWithNode.peek().curMin));
    }
}

public void pop() {
    if (minStackWithNode.isEmpty()) {
        return;
    }
    minStackWithNode.pop();
}

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

public int getMin() {
    if (minStackWithNode.isEmpty()) {
        return -1;
    }
    return minStackWithNode.peek().curMin;
}