GetMinStack.java

package gfg.ds.stack;

import java.util.ArrayDeque;

public class GetMinStack {
  private ArrayDeque<Integer> stack = new ArrayDeque<>();
  private ArrayDeque<Integer> minStack = new ArrayDeque<>();

  /**
   * we have to push for equals to only for multiple entries of minimum element.
   */
  public void push(int data) {
    this.stack.push(data);
    if (minStack.isEmpty() || minStack.peek() >= data) {
      minStack.push(data);
    }
  }

  public int pop() {
    assert !isEmpty() : "Stack is empty";
    assert stack.peek() != null;

    if (stack.peek().equals(minStack.peek())) {
      minStack.pop();
    }
    return stack.pop();
  }

  public int getMin() {
    assert !isEmpty() : "Stack is empty";
    return minStack.peek();
  }

  public boolean isEmpty() {
    return stack.isEmpty();
  }
}