Java: Pancake sort Algorithm

Java Sorting Algorithm: Exercise-14 with Solution

Write a Java program to sort an array of given integers using the Pancake sort algorithm.

Pancake sorting is the colloquial term for the mathematical problem of sorting a disordered stack of pancakes in order of size. This is when a spatula can be inserted at any point in the stack and used to flip all pancakes above it. Pancake numbers are the minimum number of flips required for a given number of pancakes. The problem was first discussed by American geometer Jacob E. Goodman. It is a variation of the sorting problem in which the only allowed operation is to reverse the elements of some prefix of the sequence.

Sample Solution:

Java Code:

import java.util.Arrays;  

public class PancakeSort
{
   int[] heap;
 
   public String toString() {
      String info = "";
      for (int x: heap)
         info += x + " ";
      return info;
   }
 
   public void flip(int n) {
      for (int i = 0; i < (n+1) / 2; ++i) {
         int tmp = heap[i];
         heap[i] = heap[n-i];
         heap[n-i] = tmp;
      }      
     // System.out.println("flip(0.." + n + "): " + toString());
   }
 
   public int[] minmax(int n) {
      int xm, xM;
      xm = xM = heap[0];
      int posm = 0, posM = 0;
 
      for (int i = 1; i < n; ++i) {
         if (heap[i] < xm) {
            xm = heap[i];
            posm = i;
         }
         else if (heap[i] > xM) {
            xM = heap[i];
            posM = i;
         }
      }
      return new int[] {posm, posM};
   }
 
   public void sort(int n, int dir) {
      if (n == 0) return;
 
      int[] mM = minmax(n);
      int bestXPos = mM[dir];
      int altXPos = mM[1-dir];
      boolean flipped = false;
 
      if (bestXPos == n-1) {
         --n;
      }
      else if (bestXPos == 0) {
         flip(n-1);
         --n;
      }
      else if (altXPos == n-1) {
         dir = 1-dir;
         --n;
         flipped = true;
      }
      else {
         flip(bestXPos);      }
      sort(n, dir);
 
      if (flipped) {
         flip(n);
      }
   }
 
   PancakeSort(int[] numbers) {
      heap = numbers;
      sort(numbers.length, 1);
   } 
 
   public static void main(String[] args) {
      int numbers[] = {7, -5, 3, 2, 1, 0, 45};
      System.out.println("Original Array:");
      System.out.println(Arrays.toString(numbers));
      PancakeSort pancakes = new PancakeSort(numbers);
      System.out.println("Sorted Array:");
      System.out.println(Arrays.toString(numbers));
   }
}

Sample Output:

Original Array:
[7, -5, 3, 2, 1, 0, 45]
Sorted Array:
[-5, 0, 1, 2, 3, 7, 45]]

Flowchart:

Java Code Editor:

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