Added Heap_sort.java (tornado-12#103)

Added Heap sort in java.
with detailed explanation

Java/Heap_Sort.java

@@ -0,0 +1,80 @@
+//Why is heap sort more efficient than selection sort?
+//Heapsort is just like selection sort, but with a better way to get the largest element. 
+//Instead of scanning all the items to find the max, it pulls it from a heap. 
+//Heaps have properties that allow heapsort to work in-place, without additional memory. 
+//Creating the heap is O(N lg N).
+
+
+
+
+// Java program for implementation of Heap Sort
+public class HeapSort {
+    public void sort(int arr[])
+    {
+        int n = arr.length;
+
+        // Build heap (rearrange array)
+        for (int i = n / 2 - 1; i >= 0; i--)
+            heapify(arr, n, i);
+
+        // One by one extract an element from heap
+        for (int i = n - 1; i > 0; i--) {
+            // Move current root to end
+            int temp = arr[0];
+            arr[0] = arr[i];
+            arr[i] = temp;
+
+            // call max heapify on the reduced heap
+            heapify(arr, i, 0);
+        }
+    }
+
+    // To heapify a subtree rooted with node i which is
+    // an index in arr[]. n is size of heap
+    void heapify(int arr[], int n, int i)
+    {
+        int largest = i; // Initialize largest as root
+        int l = 2 * i + 1; // left = 2*i + 1
+        int r = 2 * i + 2; // right = 2*i + 2
+
+        // If left child is larger than root
+        if (l < n && arr[l] > arr[largest])
+            largest = l;
+
+        // If right child is larger than largest so far
+        if (r < n && arr[r] > arr[largest])
+            largest = r;
+
+        // If largest is not root
+        if (largest != i) {
+            int swap = arr[i];
+            arr[i] = arr[largest];
+            arr[largest] = swap;
+
+            // Recursively heapify the affected sub-tree
+            heapify(arr, n, largest);
+        }
+    }
+
+    /* A utility function to print array of size n */
+    static void printArray(int arr[])
+    {
+        int n = arr.length;
+        for (int i = 0; i < n; ++i)
+            System.out.print(arr[i] + " ");
+        System.out.println();
+    }
+
+    // Driver code
+    public static void main(String args[])
+    {
+        int arr[] = { 12, 11, 13, 5, 6, 7 };
+        int n = arr.length;
+
+        HeapSort ob = new HeapSort();
+        ob.sort(arr);
+
+        System.out.println("Sorted array is");
+        printArray(arr);
+    }
+}