Retrieving the Min element in a stack in O(1) Time

Question

The reason I\'m asking this question is because I cannot see why the way I think cannot be applied to this particular question

\"How would you design a stack which,

Answer 1

Use a linked list to keep track of the minimum value which is gonna be the head.

Note that linkedlist.app= append ( we put the value in the tail ). linkedlist.pre =prepend ( we put the value as the head of the linkedlist)

public class Stack {

int[] elements;
int top;
Linkedlists min;

public Stack(int n) {
    elements = new int[n];
    top = 0;
    min = new Linkedlists();
}

public void realloc(int n) {
    int[] tab = new int[n];
    for (int i = 0; i < top; i++) {
        tab[i] = elements[i];
    }

    elements = tab;
}

public void push(int x) {
    if (top == elements.length) {
        realloc(elements.length * 2);
    }
    if (top == 0) {
        min.pre(x);
    } else if (x < min.head.data) {
        min.pre(x);
    } else {
        min.app(x);
    }
    elements[top++] = x;
}

public int pop() {

    int x = elements[--top];
    if (top == 0) {

    }
    if (this.getMin() == x) {
        min.head = min.head.next;
    }
    elements[top] = 0;
    if (4 * top < elements.length) {
        realloc((elements.length + 1) / 2);
    }

    return x;
}

public void display() {
    for (Object x : elements) {
        System.out.print(x + " ");
    }

}

public int getMin() {
    if (top == 0) {
        return 0;
    }
    return this.min.head.data;
}

public static void main(String[] args) {
    Stack stack = new Stack(4);
    stack.push(2);
    stack.push(3);
    stack.push(1);
    stack.push(4);
    stack.push(5);
    stack.pop();
    stack.pop();
    stack.pop();
    stack.push(1);
    stack.pop();
    stack.pop();
    stack.pop();
    stack.push(2);
    System.out.println(stack.getMin());
    stack.display();

}

}