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import java.util.Stack;

/**

* How would you design a stack which, in addition to push and pop, has a function min which returns the minimum

* element? Push, pop, and min should all operate in O(1) time.

*/

public class StackMin {

private Stack<Integer> mainstack = new Stack<>();

private Stack<Integer> minstack = new Stack<>();

public StackMin() {

}

public void push(Integer item) {

mainstack.push(item);

if(minstack.isEmpty()){

minstack.push(item);

} else {

if(minstack.peek() >= item){

minstack.push(item);

}

}

}

public int pop() {

if(mainstack.isEmpty()){

return -1;

} else {

if(mainstack.peek() == minstack.peek()){

minstack.pop();

}

return mainstack.pop();

}

}

public int min() {

if(minstack.isEmpty()){

return -1;

}

return minstack.peek();

}

public static void main(String[] args) {

StackMin sm = new StackMin();

sm.push(6);

System.out.println("Min after pushing 6 = " + sm.min());

sm.push(7);

System.out.println("Min after pushing 2 = " + sm.min());

sm.push(3);

System.out.println("Min after pushing 3 = " + sm.min());

sm.push(4);

System.out.println("Min after pushing 4 = " + sm.min());

sm.push(5);

System.out.println("Min after pushing 5 = " + sm.min());

sm.push(2);

System.out.println("Min after pushing 2 = " + sm.min());

System.out.println("\n Now popping....!");

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

System.out.println("Min after popping " + sm.pop() + " = " + sm.min());

}

}