Adding implementation of binary search rank

algs4/src/meatballhat.com/algs4/binarysearch.go

@@ -0,0 +1,21 @@
+package algs4
+
+func BinarySearchRank(key int, a []int) int {
+    lo := 0
+    hi := len(a) - 1
+
+    var mid int
+
+    for lo <= hi {
+        mid = lo + (hi - lo) / 2
+        if key < a[mid] {
+            hi = mid - 1
+        } else if key > a[mid] {
+            lo = mid + 1
+        } else {
+            return mid
+        }
+    }
+
+    return -1
+}