Problem3-2-extend-stack.java

public class StackWithMin extends Stack<Integer> {
  Stack<Integer> minStack;
  private int minValue;
  
  public StackWithMin() {
    super();
    
    minStack = new Stack<Integer>();
  }
  
  public Integer getMin() {
    if (minStack.isEmpty())
      return Integer.MAX_VALUE;
    return minStack.peek(); // get top
  }
  
  public void push(int data) {
    super.push(data);
    
    if (this.isEmpty()) {
      minValue = data;
      minStack.push(data);
    }
    else {
      if (data < minValue) {
        minValue = data;
        minStack.push(data);
      }
    }
  }
  
  public Integer pop() {
    if (this.isEmpty())
      return null;
      
    int value = super.pop();
    if (value == getMin()) {
      minStack.pop();
    }
    
    return value;
  }
}