adds more comments to heapsort

Author: kevin-wayne

src/main/java/edu/princeton/cs/algs4/Heap.java

@@ -53,11 +53,16 @@ private Heap() { }
      */
     public static void sort(Comparable[] pq) {
         int n = pq.length;
+
+        // heapify phase
         for (int k = n/2; k >= 1; k--)
             sink(pq, k, n);
-        while (n > 1) {
-            exch(pq, 1, n--);
-            sink(pq, 1, n);
+
+        // sortdown phase
+        int k = n;
+        while (k > 1) {
+            exch(pq, 1, k--);
+            sink(pq, 1, k);
         }
     }