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

// Author : Tianming Cao

// Date : 2018-02-02

/**********************************************************************************

* 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.

* Example:

* MinStack minStack = new MinStack();

* minStack.push(-2);

* minStack.push(0);

* minStack.push(-3);

* minStack.getMin(); --> Returns -3.

* minStack.pop();

* minStack.top(); --> Returns 0.

* minStack.getMin(); --> Returns -2.

*

**********************************************************************************/

package minStack;

import java.util.Stack;

public class MinStack {

public Stack<Integer> mainStack;

/**

* Call an extra stack named assistStack to store the min value.

* While we doing push operation, compare x with the top of assistStack and push the smaller value into assistStack.

* The other operations pop,top and getMin is very simple.

**/

public Stack<Integer> assistStack;

public MinStack() {

mainStack = new Stack<Integer>();

assistStack = new Stack<Integer>();

}

public void push(int x) {

mainStack.push(x);

if (assistStack.isEmpty()) {

assistStack.push(x);

} else {

assistStack.push(Math.min(x, getMin()));

}

}

public void pop() {

mainStack.pop();

assistStack.pop();

}

public int top() {

return mainStack.peek();

}

public int getMin() {

return assistStack.peek();

}

}