Add notes

1 files changed, 31 insertions, 0 deletions

diff --git a/doc/unit/10/README.md b/doc/unit/10/README.md
index e69de29..1c6b645 100644
--- a/doc/unit/10/README.md
+++ b/doc/unit/10/README.md
@@ -0,0 +1,31 @@
+# Heaps
+
+Eytzinger's method can represent a complete binary tree as an array.
+
+`2i + 1` and the right child of the node at index `i` is at
+index `right(i) = 2i +2`. The parent of the node at index `i` is at index
+`parent(i) = (i-1)/2`.
+
+```java
+class BinaryHeap {
+  T[] a;
+  int n;
+
+  int left(int i) {
+    return 2*i + 1;
+  }
+  int right(int i) {
+    return 2*i + 2;
+  }
+  int parent(int i) {
+    return (i-1)/2;
+  }
+}
+```
+
+A `BinaryHeap` uses this technique to implicitly represent a complete
+binary tree in which the elements are `heap-ordered.`
+
+heap-ordered: The value stored at any index `i` is not smaller than the value stored at index `parent(i)`, with the exception of the root value, `i = 0`.
+The smallest value in the priority Queue is at position 0.
+