Question

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Practice problem on stacks and queues. Thanks :)

Write a method called divideStack that takes a stack of integers as a parameter and divides it into negatives and positives.The numbers in the stack must be arranged so that all negatives appear on the bottom of the stack and all positives appear on the top. The order of the numbers does not matter as long as all negatives appear lower in the stack than all positives. ***YOU MUST USE A QUEUE AS AN AUXILIARY STORAGE***

Answer 1

Complete JAVA program to demonstrate divideStack method:-

import java.util.Queue;
import java.util.Stack;
import java.util.LinkedList;

class Main {
  // Method to divide stack into positives and negatives using a queue
  public static void divideStack(Stack<Integer> st){
    Queue<Integer> q = new LinkedList<Integer>();
    int numOfNegative = 0;// Number of negative numbers in the stack
    // Loop till stack becomes empty
    while(!st.isEmpty()){
      int val = st.pop();// Pop a value from the stack
      if(val < 0)// If popped value is negative
        numOfNegative++;
      q.add(val);// Insert value into the queue
    }
    
    while(numOfNegative > 0){
      int val = q.remove();// Remove a value from the queue
      if(val < 0){// If value is negative
        st.push(val);// Push negative values first
        numOfNegative--;
      }
      else
        q.add(val);// Insert positive value back in the queue
    }

    // Now, only positive values are remaining in the queue
    // Push all positive values into the stack
    while(!q.isEmpty()){
      int val = q.remove();// Remove a value from the queue
      st.push(val);// Push positive value into the stack
    }
  }

  // Main method to demonstrate divideStack method
  public static void main(String[] args) {
    Stack<Integer> st = new Stack<Integer>();// Create a stack

    // Push values into the stack
    st.push(-5);
    st.push(4);
    st.push(6);
    st.push(-9);
    st.push(-7);
    st.push(11);
    st.push(-12);

    System.out.println("Stack before calling divideStack(): " + st);

    divideStack(st);

    System.out.println("Stack after calling divideStack(): " + st);
  }
}

OUTPUT:

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