MinStack.java

import java.util.Comparator;
import java.util.PriorityQueue;

class MinStack {
 int[] stack;
 private int index;
 private PriorityQueue<Integer> pQueue;

 public MinStack() {
 stack = new int[20];
 index = 0;
 pQueue = new PriorityQueue<>(Comparator.naturalOrder());
 }
 
 public void push(int val) {
 stack[index++] = val;
 pQueue.add(val);
 }
 
 public void pop() {
 if (index == 0) 
 return;
 
 pQueue.remove(stack[index-1]);
 stack[--index] = 0;
 }
 
 public int top() {
 if (index == 0) 
 return 0;
 
 return stack[index-1];
 }
 
 public int getMin() {
 if (pQueue.isEmpty()) 
 return 0;

 return pQueue.peek();
 }
}

/**
 * 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();
 */