heap Sort.java

/*A heap is a tree with some special properties, so value of node should be greater
than or equal to(less than or equal to in case of min heap) children of the node and
tree should be complete binary tree */

import java.util.*;
 
public class HeapSortMain {
 
   public static void buildheap(int []arr) {
      
    /*
     * As last non leaf node will be at (arr.length-1)/2 
     * so we will start from this location for heapifying the elements
     * */
    for(int i=(arr.length-1)/2; i>=0; i--){
           heapify(arr,i,arr.length-1);
      }
   }
 
   public static void heapify(int[] arr, int i,int size) { 
      int left = 2*i+1;
      int right = 2*i+2;
      int max;
      if(left <= size && arr[left] > arr[i]){
         max=left;
      } else {
         max=i;
      }
 
      if(right <= size && arr[right] > arr[max]) {
         max=right;
      }
      // If max is not current node, exchange it with max of left and right child
      if(max!=i) {
         exchange(arr,i, max);
         heapify(arr, max,size);
      }
   }
 
   public static void exchange(int[] arr,int i, int j) {
        int t = arr[i];
        arr[i] = arr[j];
        arr[j] = t; 
   }
 
   public static int[] heapSort(int[] arr) {
     
      buildheap(arr);
      int sizeOfHeap=arr.length-1;
      for(int i=sizeOfHeap; i>0; i--) {
         exchange(arr,0, i);
         sizeOfHeap=sizeOfHeap-1;
         heapify(arr, 0,sizeOfHeap);
      }
      return arr;
   }
 
   public static void main(String[] args) {
      int[] arr={1,10,16,19,3,5};
      System.out.println("Before Heap Sort : ");
      System.out.println(Arrays.toString(arr));
      arr=heapSort(arr);
      System.out.println("=====================");
      System.out.println("After Heap Sort : ");
      System.out.println(Arrays.toString(arr));
   }
}