MinStack.java

//Design a min stack
//Time Complexity : O(1)
//Space Complexity : O(n)

import java.util.Stack;

class MinStack {
 Stack<Integer> minStack;
 int min;
 public MinStack() {
 this.minStack = new Stack<>();
 this.min = Integer.MAX_VALUE;
 }

 public void push(int val) {
 //Push previous minimum & new minimum to stack when minimum changes
 if(min >= val) {
 minStack.push(min);
 min = val;
 }
 minStack.push(val);
 }

 public void pop() {
 if(min == minStack.pop()) {
 min = minStack.pop();
 }
 }

 public int top() {
 return minStack.peek();
 }

 public int getMin() {
 return min;
 }
}