add code of 278

Author: Li Li

Algorithms Basics/Chapter 6. Binary Search/278. First Bad Version.cs

@@ -0,0 +1,20 @@
+/* The isBadVersion API is defined in the parent class VersionControl.
+    bool IsBadVersion(int version); */
+
+public class Solution : VersionControl {
+    public int FirstBadVersion(int n) {
+        if (n == 0) return 0;
+        int start = 1;
+        int end = n;
+        while (start + 1 < end) {
+            int mid = start + (end - start) / 2;
+            if (IsBadVersion(mid)) {
+                end = mid;
+            } else {
+                start = mid;
+            }
+        }
+        if (IsBadVersion(start)) return start;
+        return end;
+    }
+}